Yeah, that's the big question. I disagree about wrist being the main source of power. Perhaps my understanding is incorrect, but I believe that rotation about the wrist joint is large passive - the elbow slows down and the racquet, being possessed of potential energy, then accelerates around the wrist/forearm axis. The racquet rotates the wrist in other words, although fine adjustments could be made by wrist muscles if necessary. The timing of this proximal deceleration could then determine the orientation of the racquet face at contact, and thus the direction of the shot. If the deceleration begins slightly earlier, the shot will go cross-court; if the deceleration begins slightly later, the shot will go DTL or inside-out. How many milliseconds from initiation of elbow deceleration to impact? It would seem that the cross-court timing would lead to greater racquet-head speed at impact due to longer period of acceleration. Also Toly, what do you think about the possibility that the racquet is rotating the forearm into pronation, as per the "Tennis Racquet Theorem," rather than the other way around?