What determines if a pitch is considered illegal when there are no runners on base?

In the context of baseball, a pitch is considered illegal if the pivot foot is off the pitcher's plate during the pitch when there are no runners on base. The rules state that the pitcher must maintain contact with the pitcher's plate with their pivot foot during the delivery of the pitch to ensure that the pitch is legal.

If the pivot foot loses contact with the plate, it can result in a violation known as a "illegal pitch." This rule is intended to maintain fairness and consistency in the pitching mechanics, and it is crucial for the integrity of the game.

While other options relate to various aspects of pitching and batting, they do not pertain to the situation outlined in this context. For instance, stepping off the pitcher's plate, the pitch entering the strike zone, or whether the batter swings or not, do not impact the legality of the pitch in accordance with this specific rule when no runners are present. Thus, the correct determination of an illegal pitch hinges specifically on the status of the pivot foot relative to the pitcher’s plate.