Originally Posted by mntlblok
As I've pondered this subject over the past year or so, I've come to a nearly identical conclusion. That is, the "later" pronation, the more "sideways" racket path, the lower and more "to the left" contact yielding the topspin.
Hadn't thought about the difference in the shoulder turn, but that makes a lot of sense, too, and maybe helps explain why even some of the better local women players can't hit the topspin serve, and often bring that right foot over to the right of the left in their "pinpoint" service motions.
FuzzyYBs has an excellent video on how to hit a topspin serve that includes a slow motion video taken from above the same server hitting a first and second serve. This clearly demonstrates the shoulder turn and difference in racket path. The first serve is more toward the target and the second serve the server turns more away from the ball and across. At contact the shoulders are less square to the target. Just doing this will put more spin on the ball.