but as Hogan said fundamentally the downswing needs to be on a shallower plane than the backswing and that requires vertical movement of the hands relative to the plane.
Just to be clear I do not think that you meant that Hogan said one must drop the hands down relative to the plane to start the downswing? I only recall Hogan saying that the first move down is started with the lower body and the hands go along for the ride... Perhaps you are saying that the body move drops the hands down??? Or are you saying that there is a separate movement of the arms to drop the hands down below the plane in Hogan's swing??? Perhaps something else?