Note that the archer's bow is evident for some servers at the trophy phase while others do exhibit the bow until the racquet head drop (after the trophy position).
Originally Posted by netguy
Roger doesn't push his hip forward. He pushes the balls of his back foot against the ground (where more than 50% of his weight is resting at that moment) and as a result his hips move forward. However, the more important thing to put attention to on the photo is that Roger's knees are not at the same high level at that moment which is key for achiving balance and power.
I'm not sure that I buy the premise that Roger has more than 50% of his weight on his back foot at the trophy phase of his serve. The bottom line is that his hips do move forward as the racquet head drops from the trophy position. Roger's "bow" is mild -- his is not really the best example of an archer's bow. The bow is usually much more evident with players that utilize a narrower platform, like Roddick, and players who employ a pinpoint stance. Sampras had a bit more of a bow that Federer but probably not as much as Roddick or Ivanisevic and other pinpointers.