Rule 2.00 defines the Infield Fly as, “a fair fly ball (not including a line drive or a bunt) which can be caught by an infielder with ordinary effort, when first and second, or first, second, and third bases are occupied before two are out. The pitcher, catcher, and any outfielder stationed in the infield on the play shall be considered infielders for the purpose of this rule.” It goes on to state that “[t]he ball is alive and runners may advance at the risk of being caught or retouch and advance after the ball is touched, the same as on any fly ball. If the hit becomes a foul ball, it is treated the same as any foul ball.”
Here are the key elements in understanding the Infield Fly Rule:
1) There must be less than 2 outs;
2) There must be runners on first and second OR first, second, and third;
3) The fly ball cannot be a bunt or a line drive;
4) An infielder must be able to catch the ball with ordinary effort.
The Infield Fly only calls the batter out. It does NOT create a dead-ball situation. Runners are allowed to advance at their own jeopardy the same as any other fly ball. If caught, the runners must retouch the base or risk being called out on appeal. If uncaught, the runners may run or choose to stay on their base, but if they run they have to be tagged out as they are no longer forced to run.
The purpose of the rule is to protect the runners on base. This rule IS NOT supposed to be a free gift to the defense. The batter is ruled out so the runners are no longer forced to advance if the ball falls untouched. Without this rule, the defense could allow the ball to fall untouched to the ground and turn an easy double-play because the runners have to tag up for the fly ball.
The reason there has to be runners on first and second or first, second, and third, is because there must be at least two runners on base subject to a “force play.” Otherwise, the defense can gain no advantage by allowing the ball to drop. Also, there must be less than two outs, or it would be just as easy to catch the fly ball as let it drop and get the force out.