Is it true that the pitcher's hands separate over the pitching rubber?
They often do, although they don't always. For many, breaking the hands synchronizes with the lower body movement (breaking the hands occurs at the same time as your lead leg starts to move towards the plate).
When pitching from the stretch, all parts of your body are in front of the rubber - don't think the hands will break over the rubber in that case.
I teach hands breaking in front of the rubber from stretch and wind up. I like the hips to be well forward of the rubber before explosive hand break.