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 windup position when he puts his pivot foot on the pitching rubber and has both shoulders facing the batter to some degree. When starting his delivery from the windup position, a pitcher has the option to take a step back or to the side with his free foot. The pitcher then turns his pivot foot to be parallel to the pitching rubber, 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 windup position is rarely used with runners on base, as the length of time needed to complete the windup makes pitchers more susceptible to stolen bases if using it with men on base.