Originally Posted by toly
The wrist can rotate the racquet about two orthogonal axes. Djokovic palm is horizontal, thus wrist deviations can rotate the racquet about vertical axis only. In picture 1, from above view, Djokovic rotates the racquet counterclockwise by using wrist ulnar deviation and racquet moves forward. If he used wrist radial deviation, picture 2, the racquet would move backward, away from the ball.
Toly, in this picture, the racquet head is almost surely also moving upwards and across, in addition to the forward direction. Wrist movement, if any, would most likely be a (passive) flexing movement which (due to the western grip) moves the racquet mostly upwards (one reason why western grips yield more spin - an eastern grip would result in more forward movement when the wrist flexes). The forward movement in this picture most likely comes from a number of sources, a significant component coming from ISR (which also moves the racquet up and across), and the rest coming from the shoulder and other sources. Ulnar (or radial) deviation, if any, would be insignificant, and probably only done for fine control of the racquet head.
A case can be made that all active movements of the wrist are for purely control purposes, and any contribution to power is incidental. But we've discussed this many times before...