Both types of pitching position have their strengths and weaknesses. Compared to the set, the windup has a relatively slower execution, and it is therefore better suited for situations in which there are no baserunners or when the lead runner is on third base, because it is difficult to steal home plate. Conversely, a pitch from the set, having a relatively faster execution, is preferred when there are baserunners. Faster execution is important to prevent stolen bases.
However, some pitchers, particularly relief pitchers, are more comfortable pitching from the set position and thus use it regardless of the situation. Whether a pitcher starts from the windup or the stretch is all about what feels best for their own personal preference. The reason more than half of pitchers start from the windup position is because that type of motion gets a little bit more of a rhythm going before pitching.
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