The over and back rule in men's lacrosse is an essential regulation that governs possession and ball movement across the midfield line. Once a team has successfully cleared the ball into their offensive half and established possession inside the restraining box (goal area), they are not allowed to send the ball back into their defensive half unless a defensive player causes the ball to cross the centerline. If the offensive team is the last to possess and touch the ball before it crosses back over the midfield line, a turnover is called, and possession is awarded to the opposing team.
The rule is designed to keep the pace of play high, encourage offensive action, and prevent stalling tactics where teams would otherwise retreat into their defensive half to reset play unnecessarily. The over and back rule aligns lacrosse more closely with other sports like basketball, which has a similar backcourt violation rule.