Impulse is the mysterious, invisible magic underlying A-level tennis. It is the reason that A players hardly ever miss no matter how hard they hit. It is the secret behind hitting placement winners on the run, on the reach, behind the back or between the legs. It is the wonder you feel when a talented beginner with no apparent stroking ability reduces you to a pile of self-hating excrement. It may not be the elusive "secret of tennis", but it is the closest thing to it that I have found and unlike superior neurokinetics or a favorable fast twitch-slow twitch muscle fiber ratio, impulse is magic that even the untalented can muster to their side.

Controlling with the Swing

To be clear - anytime you need to get control of a tennis ball for any reason; be it to get a serve in the box, keep a volley out of the net and inside the baseline, direct a lob up, so it doesn't sail over the baseline. That's right; every time you make contact with the ball, you must apply impulse. Just swinging at the ball - swinging low-to-high, controlling the racket face, varying the racket head speed - may change ball speed and reflect the ball grossly in the direction of your opponent's court, but it does nothing to bring the ball under your control. It doesn't matter how hard or soft you hit, where your racket face is pointing, what direction your racket is moving; as far as the ball is concerned, it is just hitting a wall and will come off that wall in a direction of its choosing. How many times have you watched a ball you thought was well hit make a beeline for the alley or the base of the net and think "I know I did not aim that ball there!"

Here is the deal: When you throw your racket face at the ball (without impulse), there are three things about the racket and three things about the ball that determine the ultimate flight path of the ball off the strings. Each is very important because together they determine if the ball clears the net and whether it lands in or out. The interaction of the racket with the ball is more complex than people think. For example, the direction the ball flies off the racket is almost NEVER in the direction the face of the racket is pointing. This one fact might explain why so many of your volleys end up in the base of the net. It's not your fault - exactly - you are probably aiming the racket face at that sliver of space over the top of the net through which the ball can pass and still fall in. If the ball were coming in perfectly perpendicular to your racket and with no spin, you probably would have hit a nice volley. Unfortunately, the ball is coming down at a 45-degree angle with back-spin both of which tend to make it come off the racket at an inferior angle. But wait! How were you to know what the spin was on the ball!? Or what angle it was approaching? Or how fast? How can one be unerringly consistent with all of these variables to consider and compensate for?

Fear not, for there is a solution to this dilemma, and it is called impulse. More on that later, but first lets list the factors affecting the flight path of a ball hitting your racket:

Ball Related Factors
1. Incoming Ball Velocity - Incoming ball speed contributes as much to the pace of the ball coming off the racket as does the racket head speed,so it is difficult to predict and control the depth of your return.
2. Angle of attack of the ball relative to the racket face - All other things being equal if the ball approaches the racket from an angle of 30 degrees down and 10 degrees left it will leave the face 30 degrees down and 10 degrees to the right. The rule is "The angle of incidence equals the angle of reflection." This law is convenient when the ball is coming up from the ground, and you want it to continue to go up over the net. It is less convenient on your first volley when the ball is coming down towards your ankles, but you need it to go back up over the net or when the ball comes crosscourt and you want to redirect it down the line. That is the source of the rubric "Don't try to change the direction of the ball!" which is, of course, nonsense; A-players do it all the time.
3. The spin on the ball - I used to play a woman who hit the most viciously sliced groundstrokes you can imagine. Her ball traveled in a flat trajectory, so she had to aim it no more than 3 inches above the net to keep it in, but the ball was nearly impossible (for me) to volley. No matter how perfectly I would set my racket the ball would end up in the middle of the net. That is because the spin of the ball greatly influences its direction of flight off of the strings. A topspin ball will tend to pop up off your strings and a slice ball will dive. If you think you can predict the spin of the ball, then remember that when a ball bounces it changes its spin. A flat ball will gain moderate topspin when it bounces (it rolls a bit), whereas both heavy topspin and slice shots lose spin when they bounce. The bottom line is you don't know how much spin a ball has so you can't know or compensate for the spin when you address the ball.

Racket Related Factors
1. The velocity of the racket head- The easiest way to visualize this factor is to imagine that the racket is not moving but the ball is. Imagine you are an ant standing on top of the frame of the racket head watching the collision below. THe apparent speed of the ball would be the speed of the racket plus the speed of the ball. This is because velocities are always relative.
2. The direction racket face is facing - We all imagine that all we have to do is point the racket face at the target and we have gained mastery over the ball. Sorry to disappoint, but the orientation of the racket face has very little to do with where the ball ends up. It is no more important than the angle the ball is coming into the racket. If the ball is coming down relative to the racket, it still wants to come off of the racket in the direction of the ground. You could open up the racket face to compensate and this might help, but if you overdo it even a few degrees the ball will end up going over the fence.
3. The direction the racket is moving - Again this changes the relative angle of attack of the ball and racket. If you swing up towards a falling ball and open the racket face, you can reduce the ball's angle of attack to zero, but to what end? If you aren't hitting a nice gentle lob, then that ball is gone, gone, gone!

Controlling with Impulse

So...how the heck does one seize control of that little ball and send it reliably to its target? The answer is: Be impulsive.

Impulse occurs when we apply a force to an object over a discrete period. When you are trying to get a sled going you don't hit it; you push it. That sounds pretty trivial until you compare pushing an object verses colliding with it. In a collision, it is much more difficult to predict the direction that an object will take after impact. Conversely, if you apply a steady force to an object over time you have a pretty good idea where it will go; in the direction of the force. The greater the force and the longer the time, the faster the ball leaves the racket and the more the final flight path will coincide with the direction of the impulsive force. The real magic of impulse is that it is directional. The increase in momentum of the tennis ball is exclusively in the direction of the applied force. It essentially erases all of the racket and ball factors listed above. That means that the flight of the ball is under your control - and if you apply force consistently your shots will be consistent. To take advantage of this miracle of physics all you need to do is to focus on the applied force at the moment-of-contact, not on the direction of the rackets face, its head speed or path to the ball. The miracle of impulse explains why the strokes of the top pros look so odd; why the racket face often seems to flail all over the place, not point directly at their targets. Top athletes instinctively know how to deliver directed impulse into the ball to achieve both power and control. That instinct has taught them the uncanny value of "snap".

Generating Snap

Snap is the source of impulse, and as such it is the source of all control in tennis. Snap is not a sound, exactly. It is more of a feeling. It is that quick, tiny, subtle, violent motion that precedes contact with the ball. It is a motion we use when we kill flies with a flyswatter, pound nails, throw a ball, toss a Frisbee (properly) or flick paint off of a brush. Generating snap usually requires a brief reverse or counter-force (a lock or backswing proper) followed by a strong forward driving force (a load) ending with a second, more subtle counter-force (an explode) that slows, stops or reverses the forward acceleration introduced in the load - all working through a flexible joint such as the wrist. The purpose of the lock force it to get the racket accelerating in the wrong direction - the direction opposite of the one you want the ball to go. Then the forward load force pulls the wrist towards the ball but temporarily leaves the racket head behind owing to its inertia. The inertia of the of the racket head tends to bend the wrist backward away from the ball stretching the forarm muscles. As they are stretched, the forarm muscles contract internally, tightening like variable tension springs. That tension is the key, and the process that stores it is called the "stretch-shortening cycle". Now comes the magic. If that tension is still present when the racket face contacts the ball, and it is unopposed by the inertia of the racket, those 'stored forces' will be injected directly into the ball as impulse, contributing to both power and control over the flight path of the ball. This is the purpose of the 'explode'. This last counter force decelerates the arm-racket complex, destroying the inertial force of the racket and releasing the the forces stored in the forarm. This can be done either by suddenly discontinuing the forward driving forces, as in a volley, or to 'slam on the bakes' by pushing back against the driving forces of the load. This is what you are doing if you are properly 'stepping into the ball' on your groundstrokes. The drirving force must be 'turned off' so the stored force is no longer 'bottled up' and will be automatically released into the ball. The magic part is that all of this can happen without any apparent movement of the racket, since it is acceleration, not velocity, that is important for control.

Note that the racket head may (as in a groundstroke) or may not (as in a volley) be moving as it encounters the ball, but a highly directional force is being delivered directly into the ball at the moment of contact giving you ultimate authority over the ball's fate. Any constant velocity motion of the racket head is irrelevant in this exchange, so this type of control works as well in a short stroke such as a volley as it does in a full groundstroke or serve. Note also the importance of not trying to drive the racket through the ball to achieve maximum pace or spin. If there is an acceleration force from the shoulders present at the moment of contact, that will be the principal source of force and therefore impulse. The stored forces in the forearm will not be released into the ball since the inertia of the racket head fed by the acceleration of the shoulders will continue to keep the stored force stored until long after the ball has left the racket.

OK, I hear you. You are asking "So why don't I do what my teaching pro told me to do and just guide the ball to the target with the same force I use to accelerate the racket and generate spin? I just need to improve the shape of my swing!" Well, that is what you are likely doing now, and how is that working out for you? The problem here is that the direction you want the ball to go does not coincide with the direction you need the racket head to travel in order to address the ball, generate spin, and add pace. You first have to get the racket head from wherever it is in your backswing to that point in spacetime that represents the ideal point of contact(POC) with the ball. You will not know where that point is until the ball gets pretty close, so you can't adjust your backswing to match the POC. If you draw an imaginary line from your racket face in the backswing to the POC, that determines the direction of corce needed to arrange a meeting between the racket and the ball, and I can virtually guarantee that the line does not correspond to the direction you want the ball to go. Even if it did correspond, you also have to add spin to the ball, and that usually entails a flight path of the racket head that is either low-to high (for topspin) or high to low (for underspin) across the back of the ball. In fact that direction mostly determines the flight path of the racket head and addressing the ball on the way is simply a matter of timing. So it is pointless to try to push or swing the ball into your opponent's court. You must use stored forces to inject impulse into the ball if you want any control at all over the ball. These are forces created early in the stroke, stored in the forearm and delivered to the ball at the moment of contact and after the gross forces used to generate pace and spin have dissapated. Note that the racket head may still be moving as a result of those gross forces when impulse is released, but it should not be accelerated by any force except the stored forces. In fact, swinging in the direction you want the ball to go is just wrong. It is bad for your game and your elbow. Using impulse through the application of snap (or lock and load) is the only reliable source of control in tennis (and, I suspect, a host of other sports).