But that's not all that is different in his stance. As Roddick moves the front foot back, his rear knee comes forward so that his weight shifts more forward onto the toes of his rear foot. Then he reverses course. His knee actually moves back the other way, so that his weight shifts backwards again. The result is that his weight is much more on the ball of his rear foot. He completes this shift just as both knees reach the point of maximum bend.
Notice how Roddick shifts his weight to the back foot at the maximum knee bend.
What's the effect? The distribution of his weight in the narrow stance makes it possible for Roddick to push off with both feet. He gets a very deep knee bend--probably equal to any player, including Sampras. And this knee bend combined with the push from both feet gives him tremendous leg drive. Unlike most players, he takes advantage of that deep knee bend in both legs. (Interestingly, another player who does this appears to be Maria Sharapova).
Leg Drive and the Stances
There has been a lot of discussion, in tennis coaching and also in the Forum on Tennisplayer, about the role of the stance in creating leg drive. Check out Bruce Elliot's great article on the Power Serve, in which he finds pluses and minuses to both stances. You can also compare it to my article in the Advanced Tennis section for an argument that the platform stance is superior.
Obviously, there have been (and will be) great servers using both stances. But one of the main claims about the advantages of the pinpoint doesn't hold up if we scrutinize the high speed video carefully. This argument is that players with the pinpoint stance automatically push with both feet and therefore get higher off the court and further upwards to the ball.
It's commonly believed, but it's not true. The video shows that the majority of pinpoint players don't really push off the back foot, or at best they get a minimal additional push. Some pinpoint servers like Greg Rusedski slide the back foot up until they are standing on the tips of their toes. This makes it very difficult to push with the back foot. Try it yourself. Stand up on your tip toes and try to jump up and maximize your vertical leap. It's hard to get very far up in the air off your toes.
The feet of many "pinpoint" players like Phillippoussis leave the court at different times.
Other pinpoint servers like Mark Philippoussis don't come up on the back toes. But if you look at the timing of when the feet leave the court, you can see that the back foot isn't really pushing. This is because the back foot leaves the court well before the front foot. If you really were pushing with any force with the back foot, it would lift your whole body up off the court-. Your feet would leave the court at the same time.
Try it for yourself. Stand up and just push up hard off the balls of your back foot. You can't keep the front foot on the court, even if you try. But the pinpoint players don't do that. They leave with the back foot first with the front foot still down. What really happens is that the back foot gets pulled along by the action of the front foot and the front leg.(See the Myth of the Pinpoint Stance here)