As the wrist releases after the pull, the racket head quickly whips around in an "inside out" arc. The arc that the tip of the racket travels in is much bigger than the arc of the hand path, causing it to accelerate rapidly in a whip like action.
In this image of a fan, the two colors of the fan represent the path of the racket tip and the path of the hand. Imagine that the tip of the racket follows the path of the outer white part of the fan. The hand, however, travels in the much smaller inner tan arc. By having to whip all the way around to catch up to the inner arc, we get a whip like inside out motion of the racket head. This is how racket head speed is generated.