Originally Posted by vil
I can hit enough topspin if I need to but must admit it's not the same as WW forehand I see some guys hit. That's something I'd loved to learn. I can shadow that swing (I think) but in reality it's nothing like I'm shadowing in front of the mirror. That "pat the dog" thing, is that at the take back or follow through. I imagine you can't hit the ball with that, otherwise you hit it with the edge of the frame.
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.