As I’ve said in my lessons, when you approach the net you do 2 things.

1. Split step as your opponent hits the ball

2. Stamp your front foot and the racket down at the same time as you strike the ball

But as they say, a picture is worth a thousand words, so here is a moving picture of Andy Roddick doing exactly what I told him.

I’ve put it on loop playback so you can visualize yourself doing it while watching.