In baseball, a pitcher can commit a balk when he makes an illegal motion before or during his pitch delivery. There are many ways a pitcher can commit a balk, and any time he does so, any runners on base advance to the next base unchallenged. If a runner is standing on third base when a pitcher commits a balk, that runner is allowed to cross home plate to score, thereby awarding the runner's team a point, known as a run. Many of the most common ways to balk include arm or foot motions that the pitcher makes in any other direction than toward home plate.
Perhaps the most common way to balk occurs when a pitcher does not come to the set position before delivering a pitch. In other words, a pitcher must come to a complete standstill before delivering a pitch--his hands and feet must be completely still before throwing. Determining whether a pitcher has come to a complete stop is somewhat of a subjective decision made by an umpire, and some umpires are more attentive to such motions than others.
Another way a pitcher can commit a balk occurs if a pitcher makes a motion to first, second, or third base before delivering a pitch to home plate. Again, such a determination is subjective and relies on the attentiveness of one or more umpires. For example, if a pitcher is pitching from the wind-up position--the position most commonly used when there are runners on base--and makes a step or lean toward first base before delivering his pitch, the umpire will award second base to the runner. The pitcher's motion toward first may come in the form of a lean toward the base, a shoulder motion toward first, or a hand motion toward first base. A pitcher, for example, could not pretend to throw the ball to first base and then deliver the pitch to home plate.
A pitcher can, however, look at first base as he delivers the pitch. This is not considered a balk and is completely acceptable. However, if there is a runner on second base and the pitcher turns his shoulders toward second base before delivering the pitch, the umpire will call this a balk and award third base to the runner. A pitcher can look at second base for as long as he pleases before delivering a pitch, as long as he does not turn his shoulders or the rest of his body toward second base. Further, if a pitcher chooses to attempt to pick off the runner at second base, he must first step off the pitcher's rubber before he turns to throw to second. Otherwise, his motion will be considered a balk.