On the serve to get the feel of the correct movement of the wrist/forearm use this simple wrist watch practice. This movement does not guarantee you spin but what it does is shows you how you can accelerate the racket head more.

When his racket head is back behind him he cannot see the time on the watch and also the racket face is as if you had a drink on it the drink would slide off,whereas the average club player's racket face would be facing up to the sky and so his drink would stay on..what coaches call the good waiter postion..but no power.