Originally Posted by rkelley
PTD is at the end of the take back, right before you pull the trigger to start the forward swing. From my own experience, PTD helps you load-up the pronation on your wrist so the racquet whips up more powerfully through the hitting zone, imparting more topspin.
Here's a Federer video that shows it pretty clearly.
I've noticed that pros who tend to hit flatter, like Del Potro, don't really hit the PTD position as much.
OK, I see, it's all about accelleration of the racket head. I'll have a read about it.