According to the rules of baseball, a balk is "an illegal act by the pitcher with a runner or runners on base, entitling all runners to advance one base." The purpose of the balk rule is to preserve a balance between runners attempts to steal bases and the defense's attempts to retire them. Lax enforcement of the balk rule in the 1930s through the 1950s contributed to a sharp decline in base stealing attempts. Stricter enforcement in 1988 led to that season being known as the "Year of the Balk".
The balk rule (8.05 of the rules of Major League Baseball) is complex and technical, with 13 different actions that constitute a balk. Most balks can only be seen from a specific angle, meaning that a large percentage of the spectators at a game will not have seen the violation which caused a balk to be called. A pitcher may be charged with a balk if he:
- Starts his pitching motion without completing the pitch;
- Fakes a throw to first base;
- While standing on the rubber, throws to a base without stepping directly toward that base;
- While standing on the rubber, throws or fakes a throw to an unoccupied base, unless a runner is running toward that base;
- Makes an illegal pitch, including a quick pitch;
- Pitches while not facing the batter;
- Makes any part of his pitching motion while not touching the pitching rubber;
- Unnecessarily delays the game;
- Stands on or astride the pitching rubber without the ball;
- After assuming the windup or set position, removes one hand from the ball except in the course of making a pitch or throw to a base;
- Drops the ball while standing on the pitching rubber;
- Pitches while the catcher is not in the catcher's box;
- Pitches from the set position without coming to a complete stop.
It should be noted that if a ball is put in play on a pitch that is otherwise a balk, the balk call will only stand if, as a result of the batted ball, the batter and baserunners do not all advance at least one base. If that is the case, the balk call is negated and the results of the play allowed to stand. This is the case even if one or more of the offensive players is put out after advancing the required base. For example, with a runner on first base, the pitcher commits a balk, but the batter hits the ball safely; the runner rounds second but is thrown out at third base. In this case, the balk is negated, because both the batter and runner advanced at least one base; the runner being thrown out later in the play is irrelevant. However, if the batter put the ball in play and the runner was forced out at second base, the play would be erased and the balk call would be allowed to stand, with the runner now on second base and the batter still at bat, with neither a ball nor a strike having been added to the count.
A balk committed with no runner on base results in a ball being called, even if no pitch is made.
In 2012, the Major League Rule Committee announced that it was considering changing the rule to make a move that was heretofore legal a balk, the infamous "fake to third and throw to first" play that is successful only once in a blue moon and usually only serves as a waste of time for everyone involved. Whenever the play is attempted on the road, the home fans will inevitably yell "balk", but the play was specifically allowed in Rule 8.05: "It is possible, with runners on first and third, for the pitcher to step toward third and not throw, merely to bluff the runner back to third; then seeing the runner on first start for second, turn and step toward and throw to first base. This is legal." The Rules Committee proposed to make this illegal unless the pitcher first steps off the pitching rubber (which would make the move useless as an attempt to deceive the runner). The rule change was adopted in 2013, adding the fake throw to third and throw to first play as a callable balk.