It seems the downward momentum of the racquet dropping is not very relevant here. The reason a loop is so commonly used is because it assures a good shoulder turn and good overall coil in prep for the shot. Also, any swing has a point where the racquet changes direction; it's almost like a whip cracking, where you need to quickly move the whip in the direction opposite its motion to create the desired acceleration and RHS. The momentum may have stopped momentarily, but what's more important is Nadal's stored energy, combined with the racquet being in the correct position, makes for a technically sound shot. Again, it forces a good load (the energy from which is later converted to RHS), puts the racquet in a good position, and often helps timing.
Last edited by psv255; 11-11-2012 at 07:08 PM.