The leg drive for both the platform and the traditional pinpoint players comes mainly from the front foot. But Roddick appears to be the exception. The video shows that Roddick actually does push off with both legs. In fact the push from the rear leg appears to be about the same as the push from the front leg, or just slightly less. Roddick doesn't stand up on his back toes. He pushes off with the balls of his back foot, the way the way the other players push with their front foot. His feet leave the court at almost exactly the same point in time.
Roddick's leg drive comes from the narrow stance, a deep knee bend and the push off from the balls of both feet.
For Roddick, this push with both feet is possible because of the narrow stance. Again, try it yourself right now. Stand up and narrow up your stance like Roddick. Push off upward and to the left. You'll feel it. With the narrow stance you can actually get a good push from the back feet. You'll feel that your feet leave the court at the same time.
Once we understand how Roddick is pushing, we get a possible clue why he picks up the front foot and moves it back. It's probably related to finding just the right spacing for that narrow stance. In addition to giving him a better base to push from, it's also conceivable that picking up the foot and then putting it down actually increases the loading in the muscles compared to the equivalent stationary position (as suggested to me by Scott Riewald, the head of sports science at USA Tennis High Performance.)
However it works, there is no doubt that Roddick gets massive leg drive. If you look at how high Andy gets in the air, and how far he lands inside the court, he's at the top or near the top in both dimensions compared to the other top servers.
The Role of Body Rotation
This close up reveals how Roddick gets as far into the air--and into the court--as any player.
When it comes to another major factor in the service motion, however, Andy's stance appears to be a limiting factor. This is body rotation-- the turn away from the ball in the wind-up and then the rotation of the body back the other way into the contact. Andy does have a discernable body turn, but it's nowhere near as much as Sampras or even Roger Federer.
This is due to the positioning of his feet in the stance. The narrow stance makes it impossible. Sampras and Federer are probably the two players with the greatest amount of body rotation. They both start with much more of an offset between the back and the front foot, and then turn away from the ball on the line of this stance. (To see my Sampras stance article, Click Here.) This gives them both more rotation forward into the hit.
But this rotational pattern away from and back to the ball takes time. The rhythm of Andy's serve is way too fast for that. This is due to his relatively low toss and fast wind-up. If we count the frames in the high speed video, it takes Andy less than a second to reach his maximum turn position away from the ball. By comparison, it takes Sampras about 1-1/2 seconds. From the turn it takes Pete another second to rotate back the other way and reach the contact. Andy goes from his turn position to the contact in two-thirds of that.