A pitcher is restricted to a certain set of motions and one of two basic pitching positions before and during a pitch; if these regulations are violated with one or more runners on base, an umpire may call a balk.

With a runner on base and the pitcher on or astride (with one leg on each side of) the rubber, under Official Baseball Rules, it is a balk[2] when the pitcher: *switches his pitching position from the windup to the set (or vice versa) without properly disengaging the rubber; *while on the rubber, makes a motion associated with his pitch and does not complete the delivery; *when pitching from the set position, fails to make a complete stop with his hands together before beginning to pitch; *throws from the mound to a base without stepping toward (gaining distance in the direction of) that base; throws or feints a throw from the rubber to an unoccupied base, unless a play is imminent; steps or feints from the rubber to first or third base without completing the throw (doing so to second base is legal); delivers a quick return, a pitch thrown right after receiving the ball back, with intent to catch the batter off-guard; drops the ball while on the rubber, even if by accident, if the ball does not subsequently cross a foul line; while intentionally walking a batter, releases a pitch while the catcher is out of his box with one or both feet unnecessarily delays the game pitches while facing away from the batter; after bringing his hands together on the rubber, separates them except in making a pitch or a throw; stands on or astride the rubber without the ball, or mimics a pitch without the ball; or throws to first when the first baseman, because of his distance from the base, is unable to make a play on the runner there. delivers a pitch during a squeeze play or a steal of home, if the catcher or some other player steps on or in front of home base without possession of the ball, or touches the batter or his bat. The ball is dead, the batter is awarded first base, the pitcher is charged with a balk, and the run scores. (rule 7.07)