Balk

The balk rule exists to prevent a pitcher from deceiving baserunners with an incomplete or misleading motion. Examples include starting a pickoff throw and stopping partway through, or beginning the pitching motion and not delivering to the plate. One common balk trigger is failing to come to a complete, discernible (clearly visible) stop in the stretch position before delivering. Others are stepping toward a base without actually throwing there and dropping the ball during the motion. So is making any motion associated with pitching without completing the pitch.

Because the rule covers so many specific, small mechanical details, balks are often called on pitchers who are not trying to deceive anyone. Examples include a young pitcher rushing a delivery, or a pitcher whose set position is technically incomplete. With no runners on base, the same illegal motion is simply a "no pitch" or a ball, since there is no runner to protect. The balk penalty only applies with a runner on base to advance.