As an occasional softball umpire, not to mention an all-around knowledgable guy, people often ask me what the Infield Fly Rule is. As a sports official it's important to know not only what the rules are, but why they are. Moreover, knowing the reason behind the rules makes them easier to remember as well. That is especially true in the case of the Infield Fly Rule. So here's my explanation. Once you understand what it's supposed to prevent, it's easy to remember what it does and when it applies.
The Infield Fly Rule prevents defenses from getting two cheap outs instead of one by intentionally not catching an easy pop-up. Suppose there are runners on first and second. The batter hits a pop-up to the shortstop. The fielder could easily catch the ball and the runners would have to go back. Or he could let the ball drop and the runners would have to go forward. The runners are trapped. If they don't move, the defense will let the ball drop and then throw to the bases they didn't run to. If the runners anticipate this and run, the defense will catch the ball and then throw to the bases they left. This is a cheap tactic.
The Infield Fly Rule protects the runners by saying: When there are runners on first and second, and fewer than two out, and the batter hits a fair fly ball that could be caught with ordinary effort by an infielder, the batter is automatically out even if the ball isn't caught.
Now the runners don't have to run because the batter is already out, so they're not forced to advance. There's no way for the defense to make them have to run. Making the batter out ironically helps his team.
The rule applies when there are at least TWO runners forced to advance. That means runners on first and second, with or without a runner on third. It doesn't apply when there's just a runner on first. The defense could go ahead and play their to-catch-or-not-to-catch tactic, but they'd only get that one runner out, not two, assuming the batter runs to first like he's supposed to. The rule never applies with two outs, because the defense wouldn't be trying to get two more outs, they'd just catch the ball for the third out. So if they drop the ball, it must have been unintentional.
There are fine points of the rule that every umpire learns. If the defense does catch the ball, everything turns out exactly the same as if the rule didn't apply. The runners still have to get back to their bases when the ball is caught.* Also, the ball has to be a fair ball for the rule to apply. If the third baseman lets it land in foul territory, it's just a foul ball, not an out. The ball doesn't have to be caught by an infielder, and it doesn't even have to land in the infield, as long as an infielder could have caught it with ordinary effort. "Ordinary effort" means essentially a routine pop-up. If an infielder has to run a long way to catch it, it probably wasn't routine and it's not an infield fly. Most rulebooks have a similar rule that applies to line drives - if an infielder intentionally knocks a line drive to the ground instead of catching it, it's treated similarly to an infield fly.
The Infield Fly Rule is in every rulebook at every level of both baseball and softball that I've ever encountered. If you go to a baseball game and see runners on first and second with fewer than two out, you can see the umpires signaling each other to remind themselves that the rule is in effect. They usually look at each other and touch the brims of their caps. They do this even at the major league level.
*More accurately, runners only have to "tag up" once a ball is touched, not caught. Otherwise an outfielder could run in to the infield while juggling a fly ball, finally catching it when he's standing at third base, so no one could advance on a sacrifice fly. In the 1800s players used to do that until they changed the rule. This piece of rules trivia has nothing to do with the Infield Fly Rule.
