I'm sure if I looked up the rule, it would state that the foot must be in contact with the rubber at time of release. But, having played baseball for many years and watched it even more, I know that the pitcher places his foot against the front of the rubber and pushes off. Depending on the motion and release point, he's not always going to be touching it. If he was doing some crazy crow hop, they'd call it. As it stands, they don't.