In baseball, the windup is a pitching position in which the pitcher's feet begin the pitching process on or in front of the rubber of the pitching mound and point towards home plate.
Before throwing a pitch from the windup position, a pitcher may take a step back toward second base or to the side toward first or third with his free leg (the non-pivot leg). Upon delivery of the pitch, the pitcher must take a step toward home plate. If the pitcher decides to throw to another base, he must first step off the rubber and then take a step toward the base in question. A pitcher "commits" to a pitch when he takes a step toward any base other than home plate or when he brings his hands together. If a pitcher fails to follow these rules when in the windup position, he may be charged with a balk.
The windup position is typically adopted by a pitcher when there are no baserunners. The set, or "stretch" position is most often used when bases are occupied as it is an easier position for a pitcher to throw to a base other than home plate. Pitchers can use either the windup or the set position at any time, but once committed to a particular position, they must deliver a pitch from that position unless they step off the rubber and reset their stance.