Here are a few excerpts from "Play Better Tennis In 2 Hours" by Oscar Wegner regarding The Wrist:
& Windshield Wiper Topspin
"...top tennis pros use a windshield wiper motion (bending the arm across to the left so the racquet face sweeps flat across in a arc in front of your face) on their forehands to produce topspin and force the ball down within the lines...Young players, as well as adults new to the game, need to build strength in the wrist and forearm of their hitting arm before attempting to use the windshield wiper motion with the ferocity of bit hitters...big hitters with topspin strokes use the wrist and forearm, as well as the biceps and pectorals, to move the racquet from below the ball all the way across to the left and down on the other side. The wrist doesn't flip forward, but the forearm rotates to the left, so the fingers move similarly to when you're waving at someone."
& Forehand Swing
"MYTH: Keep your racquet head above your wrist throughout your whole forehand swing.
FACT: Drop your racuqet head below the ball and below your hand at some point in your forehand sting."
"If your forehand feels stiff, loosen your wrist, lower your body and then lift it as you hit."
& The Serve
"...You may need to adjust the angle of your wrist, as if you were looking into the palm of your hand...to exaggerate the kick, break your wrist inward (without changing your grip) so that the racuqet is almost parallel to your chest."