The infield fly rule was put in place to make sure there is no unfair gamesmanship. The rule makes it so that if there is a pop fly in the infield, a fielder can not let the ball drop to allow for an easy double play. The rule is enforced when there is a force play at third base and there are less than two outs. When it is called, the batter is automatically out and the players on base must tag up if they wish to advance.