You are correct in thinking that once he has legally stepped from the rubber toward a base that he can continue to legally disengage forward.
Of course, any stepping motion from the rubber toward home commits him to pitch. While he must throw to 1B from the rubber, he need not throw to 2B or 3B. His step with his free foot is considered a feint, and he need not have arm motion with such a feint. Thus, adding 2+2 you can see that it now becomes legal for a pitcher to step toward 2B or 3B from the rubber---without arm motion---and continue proceeding toward that base or runner. It can appear as if he is merely running at the runner or base if his first step was legal.
Just my opinion,
Freix
|