In Situation I, what makes F1's throw to first base legal?

In this situation, a legal throw from F1 to first base requires the pitcher's action to be consistent with the rules governing a legal delivery and subsequent throw. If F1 throws without stopping, it suggests that he is maintaining his momentum and not committing a balk—an action that can occur if a pitcher hesitates or fails to deliver the ball in a timely manner. By making a quick throw to first base without a stop, F1 adheres to the flow of play which is important for the legality of the throw.

This choice underscores the importance of the pitcher's movements relative to the rules, allowing for a quick response to a baserunning threat without running afoul of the regulations. Other options may include actions that alter the legality of the throw, such as stepping back or employing techniques that could be interpreted as unnatural motion for a pitcher in that scenario. Thus, the action of not stopping before throwing is critical in maintaining the legality of the play.