RubberElbowSyndrome
Nothing in tennis - and I mean nothing - makes me swear foul oaths like a rubber elbow. Rubber elbow is an older term for a syndrome that is all too common in the world of the would-be-talented-if-they-could-be, and it occurs when you address a ball, usually a forehand, with so much craven fear and trepidation that your elbow seems to collapse just before contact with the ball. It feels like the bones have turned to soft, stinking, rubbery, goo under your skin. You contact the ball with a sickening "thud" and ball comes off your racket with absolutely no sense of direction at all and even less consideration for your fragile ego. The resulting shot is not just bad - it is putrid.
I have hit enough of these in my life that it is something of an old "friend" to me and I know it quite well. What happens is this: As the hitting arm starts to come forward into the load and lag phases, the hips and shoulders, instead of rotating faster, either rotate at a steady pace or slow down. Now, this slowdown should only occur in the explode phase, but instead, it happens slowly and tortuously as the racket head moves into the ball. Depending on the severity of the illness, stored control and spin producing forces in the forearm are either released early or never stored at all.
Early deceleration of the racket releases the pent-up forces you stored in the forearm during the load and all of the control and extra spin they might have provided vanishes into the aether. Sometimes the deceleration is so extreme that the racket head stops or starts moving backward just before contact with the ball. Backward acceleration is interesting because it obliterates impulse, stealing away any chance that the ball will even randomly take up the direction you intend for it.
As you may imagine, this syndrome is related to several psychological syndromes such as Ego Collapse, Forehandophobia and Expectation Syndrome. You are at risk for this catastrophic malady whenever error-terror motivates your ego to take control of your game away from your thalamus. The only way to prevent this issue is to make "hitting-out" part of your mantra, especially on the forehand. DO NOT overcompensate for rubber elbow by carrying acceleration through the moment of contact! That is a hot shoulder, and results in you holding onto the stored forces too long, releasing them only after the ball has left.