What do you think the palm position does? I can think of two potential things.
One, it might change the motion of the swing by activating certain muscles, or two, it might cause a specific stretch-shortening cycle in some muscles which make the pronation more vigorous (as pronation with good servers seems to clearly be a reflex), or perhaps both.
Well, that certainly clarified things.
I'd also be interested in hearing the responses of those more knowledgeable than I.