The best way to get a feel for it is to watch it in slow motion and then try it yourself. The upper arm traps against the ribcage and rotates internally. This causes the forearm and hand to start pronating, so that the fingers are behind the ball at release and the hand continues pronating into a palm down position after release.I think I have a decent handle on most of the mechanics. But, I feel like I don't really understand the release on the fastball.
The pronation after release is a natural consequence of the internal rotation of the upper arm and the transfer of those forces down through the forearm, not a forced mechanic. Trying to finish any other way doesn't really work unless you don't internally rotate or try to stop your internal rotation halfway through, so that your hand doesn't turn over. I don't think either of those are really what we are looking for.