My take on this: flexing the wrist locks it and aids acceleration of the racket coming forward at impact. At this point the wrist is very loose and passive. Since the racket whips forward at great speed, it causes the forearm to slow down before impact. This is because the maximum speed must be in the hand and not the forearm. It is a result of the backward force of the handle on the hand. The head of the racket rotating forward cause the handle to rotate backwards, slowing down the forearm.