When a pitcher's foot is on the rubber (the white thing on the mound) and the pitcher comes to the set position with his hands together, if the pitcher moves--even slightly--he must basically complete his delivery and pitch or throw to a base. He can also fake a throw to a base, but the base must be occupied (for instance a fake pick off on third to try and catch a runner on first off guard).
It is technically more complicated than that but that's the simplest way I can put it. If he wants to do anything other than pitch or throw to a base he has to take his foot off the rubber first. Think of it as a rule designed to prevent the pitcher from faking out the hitter or trying to catch the hitter off guard.
It is technically more complicated than that but that's the simplest way I can put it. If he wants to do anything other than pitch or throw to a base he has to take his foot off the rubber first. Think of it as a rule designed to prevent the pitcher from faking out the hitter or trying to catch the hitter off guard.