A balk is an illegal move made by or related to the pitcher. Balks result in a dead ball and all base runners (not including the batter) advancing one base. Balks are only called when there are one or more runners on base, otherwise, an illegal move by the pitcher would only result in a ball.
When does a balk happen?
Common circumstances in baseball that constitute a balk are the following:
With at least one foot on the pitcher's rubber, the pitcher makes a motion associated with his pitching delivery, but does not actually deliver the pitch.
The pitcher pretends to throw to first base or third base and does not actually complete the throw (unless he stepped off the rubber)
Once the pitcher steps on the rubber and throws to a base without first taking a step in the direction of that base.
While on the rubber, the pitcher drops the ball (even if it was an accident).
The pitcher intentionally and unnecessarily delays the game.
The pitcher throws what is known as a quick pitch. A quick pitch is when the pitcher delivers a pitch right after receiving the ball back or when the batter is not yet fully ready in the batter's box. A quick pitch also occurs when the pitcher does not take the required pause before delivering the pitch.
The pitcher switches his pitching position from the set position to the windup position (or vice versa) without first stepping off the pitcher's rubber.
The catcher steps on home plate or moves in front of it without the ball when a runner on third base is stealing home.
The pitcher fails to complete the pitching motion with set or windup while simultaneously a base runner interrupted the pitcher's delivery.
No balk is given, instead it is a "scratch" or do-over.
The pitcher does not complete his pitching motion because the batter steps out of the batter's box.
No balk is called. It is a batter's box violation.
The pitcher disengages the rubber by stepping off with his free foot.
Balk! The pitcher must step off the plate with his pivot foot.
Faking A Pitch
Baseball has lots of rules for pitchers. If a pitcher breaks one of these rules he will be called for a balk. Commonly, balks are called when the pitcher pretends to pitch, but does not actually follow through with the pitch. The reason a pitcher might do this is to deceive the base runners; he would pretend to pitch, then attempt to pick off a base runner instead of delivering the pitch, thus catching the runner off-guard. Since this is an illegal method of deception, the rule of balks is implemented.