What happens if a pitcher's arm moves slightly while in the set position with a runner on first base?

When a pitcher is in the set position and has a runner on first base, any movement of the pitcher's arm that qualifies as a violation of the rules can lead to a balk being called. A balk occurs when the pitcher makes an illegal motion that deceives the runner, preventing the defensive team from maintaining their advantage during the play. In this context, if a pitcher's arm moves slightly while in the set position, it may be viewed as an attempt to deceive the baserunner. Consequently, the umpire would call a balk, and as a result, the runner on first base advances to second base. This rule is designed to ensure fair play and uphold the integrity of the game by penalizing deceptive actions by the pitcher.