A balk is an illegal act by the pitcher when one or more runners are on base. The rule is in place to prevent a pitcher from deceiving the baserunners. When a balk is called, the ball is dead and all runners move up one base. However, the play proceeds without reference to the balk if the batter reaches first and all other runners advance at least one base.
A balk will be called when a pitcher who is on the rubber makes any motion naturally associated with his pitching delivery and does not actually deliver the ball, feigns a throw to first or third base and fails to complete the throw, or fails to step directly toward a base before throwing to that base. Once a pitcher has swung his free leg back past the pitching rubber while in the process of his leg kick, he must then deliver the ball to the plate or to second base on a pick-off attempt.
In the process of delivering a pitch, pitchers are not permitted to take a second step toward home plate with either foot or otherwise reset their pivot foot. If this occurs with one or more runners on base, the umpire can call a balk.
A pitcher can also be called for a balk if he does not come to a complete stop between the time he enters the set position and delivers the ball, or if the umpire determines that the pitcher has tried to "quick-pitch" the batter in another way. Once in the set position, a pitcher can move his head but must keep the rest of his body still and two hands on the ball until he begins his delivery. It is considered a balk if the pitcher drops the ball while touching the rubber, even if the drop is unintentional.
Additionally, balks can be called if a pitcher intentionally delays the game. This includes throwing to an unoccupied base not in an attempt to put out a baserunner, making a motion naturally associated with his delivery while not touching the rubber, and feigning a pitch on or near the pitching rubber without having the ball.
If a pitcher commits an illegal act with no runners on base, he will instead be called for an illegal pitch. In these instances, the pitch is ruled an automatic ball unless it results in the batter reaching first base via a hit, an error, a hit-by-pitch or otherwise. If the automatic ball is the fourth ball during that plate appearance, the batter will be awarded first base via a walk.
History of the rule
Pitchers used to be permitted to fake a throw to a base, and they would most commonly use a fake-to-third, throw-to-first move with runners on first and third. But this move has been ruled a balk since the 2013 season.