I think he brushes the ball (topspins shots) short to the serving line area and absorb part of the opponent's hitting power. And then he relex his wrist to start his follow through action. The time every ball touching the racquet is quite short.
turning the wrist after hitting forehand topspin shot can cause the ball to hit the ground at a shorter distance so you can keep the ball well inside the court. I do this almost all the time, especially when using new balls. but you need to get a good feel when the ball comes into contact with the racket or you will mishit all the time, as the horizontal distance whic the racket travels forward will be much shorter. definitely not for newcomers.