Here, in mid-swing, Djokovic is clearly about to hit significant topspin on his two-handed backhand. The string bed's distance below the ball ensures that it will rise to brush up the back of the ball. Novak increases that distance by dropping the racquet head below his hands. The racquet head will rise to bring the racquet's long axis to its proper position for contact, horizontal. Novak's wrists are still somewhat laid back. Their flex forward in the remaining fraction of a second before contact will contribute significantly to his racquet-head speed.
