Pitchers are permitted to use two legal pitching deliveries -- the windup position and the set position -- and either position may be used at any time.
A pitcher is considered to be in the set position when he puts his pivot foot against the pitching rubber, has both shoulders facing first (for lefty pitchers) or third (for righty pitchers) base to some degree and holds the ball with both hands in front of his body. As a result of this configuration, a pitcher in the set position points his glove-hand shoulder toward the batter and his throwing shoulder toward center field. When delivering the ball from the set position, a pitcher simply lifts his free leg up, pushes off the rubber with his pivot foot, strides toward the batter and delivers the pitch to the catcher.
The set position is often used with runners on base, as the length of time needed to complete the delivery is much shorter than the windup position. Thus, pitchers are better able to prevent runners from stealing bases from the set position. To shorten their deliveries further when pitching from the set position, some pitchers use a "slide step" to stride toward the batter rather than first using a full leg kick. Because relievers often enter the game with runners already on base, many pitch from the set position exclusively -- even when the bases are empty.