IMO, the advantage of this type of transition move is two-fold. First, it keeps a nice taut connection along the whole left side during the change in direction. The movements in the lower body are transmitted quickly and directly to the upper body, without having to take any "slack" out of the system. The first movement of the hips forces the hands to drop, from the top, into the correct position. Second, it gives the hips a head start.....more time to rotate, open up, and build speed before the upper body catches up at impact.