Ok there was another thread discussing this but I figure this post deserves its own thread. NOTE: this series of images doesn't depict what actually happens during the service motion. It is intended to break down the different rotations into simple components. I have also not yet included images that show what happens when the wrist deviates and flexes, even though these motions do make significant contributions note: ignore the fact that the torso angle would result in horrible shoulder impingement, just pay attention to the arm and ignore the torso. Part One Pronation is a technical term that refers to the counterclockwise rotation of the forearm around its own axis. See figure below: Important thing to note is that this movement contributes nothing to useful racquet head speed, in this particular anatomical configuration. In fact, if the ball is struck in the centre of the racquet, pronation in this position contributes absolutely zero to racquet head speed. This is because the racquet is simply rotating around its own axis. It is twisting, and this twisting is useless for the purpose of increasing racquet head speed. Next point: Consider the exact same rotation of the forearm, but with the wrist cocked to the side (radially deviated). Now something important happens. Because the racquet is no longer colinear (i.e. no longer in a straight line) with the forearm, it no longer only twists. It undergoes a rotation about a different axis. This allows something rather magical to happen. Consider the image below: The image depicts a rotating stick. The start position is black and the end position is red. The angle that the stick traverses is about 45 degrees. Now the key thing to understand here is that the right end of the stick travels at a much faster speed than the portions of the stick closer to the left end. This is a form of leverage, where we can generate high velocities by increasing the length of our lever. It's one of the reasons Del Potro is capable of such devastating forehands - his arms function as a very long lever, and even though he is rotating into the stroke at the same angular velocity as someone shorter than him, the end result is a faster forehand (though perhaps with less torque, but we need not worry about that). Now in the previous image that shows forearm pronation with a cocked wrist, you can see how the racquet is moving in a way that exemplifies this form of leverage. Try it at home with your own racquet, and it should become clear.