OK here is what I got from the other thread--thank you suresh:
when you yank the ball to get more power, the racquet first encounters the resistance of the impact with the ball, that is why you see a loss of speed at that moment. But because you are forcing the action with acceleration… the racquet speeds up incredibly and of course you don't intend for the racquet to hit your shoulder or you arm, so you stop it or at least you cease contracting. That is why you see a top player having more than 50 MPH on the racquet head across the body and to his left.
When you pull from the racquet rather than extending, the contraction of biceps and pectorals connects the body weight to the impact. That, together with the acceleration makes for more ball speed. If you extend, you disconnect, and you actually loose power.
Please compare to this Federer clip and see if you see any yanking going on: