To hit a 100+ mph kick serve over someone's head or make a drop volley bounce back towards the net you must apply leverage. All of these and many more tennis techniques require extraordinary racket head speed. That would not be a problem if you had the entire stroke to accelerate the racket, but you don't. The annoying requirement that you have some modicum of "control" over the flight path of the ball means that you can't start accelerating the racket head until after the Lock phase and must end acceleration before the moment of contact so that the stored control forces can be released into the ball during the Explode phase. Therefore all of the racket head speed must be developed during the Load and Lag phases - total duration about .25 seconds. During that time the racket's inertia makes it feel like it weighs 15 pounds - that is if you are only pulling the racket in a straight line. If you are also pushing the racket head around the wrist at the same time the moment of inertia of the racket head at the end of your arm and racket makes it seem much heavier. How, then, do you most efficiently get the racket "up to speed." The key is to optimize leverage during each phase of the stroke. Your joints all operate with a mechanical advantage that is less than 1.0, meaning that a large force from the muscle acts over a short distance to create a smaller force at the other end of the bone over a longer distance. Contrast this with a pry bar, where a small force over a longer distance results in a larger force over a smaller distance. Both are examples of leverage, and since our goal is to get the racket head moving as fast as possible, leverage is good. Leverage requires a rotational force or torque and a lever arm of some length. The longer the lever arm, the greater the leverage. In the context of our limbs that means we can accelerate the racket head, at the end of the lever, to a speed that is much faster than our limbs can move. So our limbs are 'step-up speed' levers, but they are also 'step-down force' levers. Early in the acceleration phase, the racket feels quite heavy due to inertia, so early on we need a short lever arm to get it going. Once it is moving, we must gradually lengthen the lever to maximize racket head speed. Varying lever length is also necessary because the torque applied to the driving end of the lever is not constant, it depends on how fast the muscle is contracting. That means that ideally, we should have a shorter lever arm to begin with and lengthen it over time, like up-shifting gears on a bicycle as we speed up. Second, power coming from the lower body arrives at the arm in a wave which builds, crests and ebbs over time, so changing leverage is one way to optimize power transfer to the ball. Increasing the length of the lever arm during a stroke is the main way that A-players develop their remarkable racket head speed and pace on the ball. There are two principal techniques that anybody can use to optimize leverage; lagging and extension.

Lagging

Lagging means dragging the racket behind the wrist during the acceleration phase of the stroke. Lagging is the polar opposite of pushing the racket at the ball. When you lag you pull on the handle of the racket along its shaft. Just prior to the moment of contact the main lever arm will slow and the wrist becomes the center of rotation so the racket will come around the wrist into the ball with excellent racket head speed. The action resembles an effective sabre-cut. Lagging also protects your arm from the ill effects of pushing the racket head into the ball.

Extension

Another helpful technique for developing power is extension. If lagging happens mostly at the wrist, extension happens at the elbow. Most full strokes start with a bent elbow and end with a straight one thus lengthening the lever arm and thereby increasing leverage as the power wave crests in the forearm. Note that as the lever arm increases in length the force at the end decreases but the linear speed of the end increases. The ability to trade force for speed is advantageous because we are trying to develop racket head speed, not force; the only forces we care about at contact are the stored control and spin forces which are stored primarily in the forearm, not sourced in the body.

Leverage and the Two-handed Backhand

Power transfer in the two-handed backhand is unique. In a particularly violent load phase, the rotational energy of the power wave is transmitted through the shoulders, dragging the racket forward. As per usual the inertia of the racket head results in a torque on both wrists, throwing the dominant (usually right) forearm into pronation and ulnar flexion and the non-dominant (usually left) forearm into supination and ulnar flexion.

The stored forces are additive in that they conspire to create forces on the racket head designed to send the ball in a direction that clears the net by 12-24 inches with lots of luscious spin and pace. In a single-handed shot, the force is all stored in the muscles of one forearm, and the force acts across two fingers of that hand, say the thumb and pinky finger - a distance of about 4 inches. The amount of torque is proportional to that distance, so in the two-hander, wherein the opposing forces are about 8 inches apart, the torque is double, and the number of muscles involved is double, so the acceleration of the racket is quadrupled. That is leverage! Superior leverage in the two-hander results in amazing consistency compared with any one-handed stroke, but that leverage is not without its drawbacks.

The superior leverage of the two-hander means that the stretch-shortening forces that are stored in the forearms during the load cannot be contained by the acceleration provided by the power wave. In the topspin forehand, for example, stored forces in the forearm are held in place for several hundred milliseconds during the lag phase. The ability to store the force is beneficial because those forces may then be released any time during that phase. That makes the timing of the forehand topspin less critical. Also, there is a long interval during which to accelerate the racket and build up racket head speed. With two hands on the racket, there are twice as many springs holding stored force. Consequently, twice as much acceleration is required if you want the inertia of the racket head to prevent the stored forces from being released. There are limits to how much acceleration can be provided by the power wave and for how long. The net result is that the two-handed backhand stroke is very short and very violent. You can produce pace, but only if the timing of the moment of contact coincides perfectly with the end of the load phase. Swing too early as often happens, and most of the stored force will have dissipated, and you don't get any pace. You still get a lot of control in that scenario, however, because the two-hander has an excess of stored control force. That is why so many 2HBH bloopers still find their way into play.

Shaping the Power Wave

So how does one change the contour of the power wave to accommodate the requirements of a high-powered short stroke such as the two-handed backhand? Since the combined forearm muscles of both arms conspire to present such a tight "spring" in which to store the control and spin forces of the stroke, the power wave must be very high in amplitude. In other words, the maximum acceleration experienced by the racket head must be very high to get the racket's inertia to drag the forearms into full pronation (dominant arm), supination (non-dominant arm) and ulnar flexion (both arms). There is no way to maintain this high level of acceleration for any length of time. By comparison, in the one-handed backhand, we can extend our arms during the stroke to increase the moment of inertia of the racket head and thereby lock up the stored force in the forearms. That way we can keep the stored forces bottled up as the power wave dissipates. This maneuver is less effective in the two-hander than it is in the one-hander due to the more rigid relationship between the racket and the body.

So we need a narrow (brief) power wave with a very high peak (large maximal acceleration). I like the less technical term "violent" to describe this requirement. In the two-handed backhand, we achieve this by supplementing the peak of the power wave as it propagates from the feet through the shoulders using another independent source of power from the shoulders and chest. Those muscles act independent of the feet and are activated just as the power wave hits the shoulders early in the load phase. They act in opposition to the inertia of the racket to pronate and radial flex the non-dominant (left) arm and supinate and radial flex the dominant (right) arm. The use of the shoulder muscles in this context carries two important requirements. First, it cannot involve the forearm muscles. The forearm muscles must be stretch shortened by the inertia of the racket head in opposition to the leverage force provided by the shoulder muscles and the rest of the power wave. Second, the leverage force must be "turned off" before the moment of contact. Otherwise, the stored forces in the forearms will not be released into the ball, and there will be no control and little spin. If the shoulders continue to act through the moment of contact, you get the "Hot Shoulder Syndrome" and the inconsistency that goes with it.