I have a player at the moment I am coaching. We are working on his serve, his takeback and trophy is all good but on the follow through his whole arm goes through in one swing, rather than the arm bending over on itself from the elbow (pronation). I have been trying to get him to throw tennis balls like a baseball player and get the feeling of the arm pronating over. I have had him stand straight on at the net and just toss the ball and work on just pronating his arm to generate all the power. He is not getting the separation of the arm where the elbow folds over like Sampras or all good servers do. Any tips on how to help him get this feeling?