A key to the exceptional power in Roddick's serve is his big knee bend, augmented by the arching of his back, both of which store lots of energy in the large muscles of the legs and torso. The racquet-head speed that ultimately delivers power to the ball comes at the end of a series of energy transfers called a kinetic chain. The kinetic chain begins with the large muscles of the legs and moves up through more large muscles in the torso to increasingly smaller body segments as it goes from shoulder to upper arm to forearm and finally to the wrist and hand, which whip the racquet forward and upward at the culmination, translating relatively slow motions of larger masses into extremely fast motions of smaller masses.
When you see a player winding up his serve with this much body arch, you can expect a good amount of topspin in the serve, and for many players, this arch would signal the heaviest of kick serves, a twist. Roddick always uses heavy topspin in his serves, even on the fastest of his first serves, and his twist serve is probably the highest-kicking serve anyone hits.