I am not qualified to get into the deep biomech discussion here, but I will make this observation. Vitually no top male server--Roddick, Sampras, Federer, you name him, has the shoulders open or parallel to the baseline at contact.
This seems to be a clear sequence in the rotation of the torso in relationship to the upward motion of the arm to the contact. The shoulders are more closed in the ad, relative to the baseline, but still closed in both courts. We are doing a study of Sampras that will measure these angles and also the hip and shoulder speeds.