Originally Posted by WildVolley
There's less pronation, or perhaps it is better to say that the timing of the pronation is different on a topspin serve. The pros usually turn more away from the court and the racket path is more across than toward the target. The racket still lags meaning that pronation (internal shoulder rotation) is need to turn the racket face to the ball, but it occurs later in the swing because the shoulder usually aren't turned as much forward. Topspin is also due to impacting the ball at a slightly lower level and more to the left.
The topspin serve needs a horizontal component like any other serve and the pronation (internal shoulder rotation) is what adds a lot of that juice.
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.
I don't think that pronating "precludes" the snapping of the wrist in that sort of combo "flexion-ulnar deviation" that causes the racket to accelerate in the plane of the string-bed for maximum spin. But, if I *do* concentrate on the pronation aspect, while I get more pace, the bounce is lower and significantly less aggravating to many opponents. *And*, my margin for error over the net drops significantly.