The infield fly rule always seems to be a point of confusion for players, coaches, and parents watching a game. The purpose of the infield fly rule is to prevent a defensive team from purposely dropping or not catching an infield fly with the intention of trying to turn a double play.
If there was no infield fly rule, the following could happen: Runners are on first and second with less than 2 outs. Pop fly is hit to the third baseman. He intentionally drops the fly ball, picks it up, touches third and then throws to second for a double play. It's an easy double play because both runners are tagging up on their bases expecting the ball to be caught.
As usual, the confusion is in the details and the combination of things that can happen. One point of confusion is that many people mistakenly think that the infield fly rule applies when there is only a runner on first base. This isn't the case. When there is no force play at home or third, the only way to turn a double play, by intentionally dropping the ball, is if the batter doesn't run to first base. In this case the infield fly rule doesn't apply and the defensive team could turn a double play. I don't think anyone will feel bad for the batter in this scenario.
So now that we know the game situation that the infield fly rule can be called, let's move on to the call itself. The main thing to remember is that the infield fly rule is a judgment call by the umpire. If the umpire determines that a player can make the catch with ordinary effort, then he/she can apply the rule. After the ball is hit in the air, you should hear the umpire yell, "Infield fly, batter is out." If you don't hear the umpire yell that, then assume the rule doesn't apply. The rule is written to give the umpire the ability to determine whether to apply the rule or not. Here is an example when the rule may not applied by the umpire. Runners on first and second. The third baseman charges toward home on a bunt coverage. The batter swings away and pops the ball up by third base. The umpire determines that the ball cannot be caught with ordinary effort, so he says nothing. The infield fly rule is not applied and the third baseman can pick the ball off the ground (if he missed it) and touch third base for a force out if the runner hasn't made it to third yet. Make sure you and your players don't assume that the infield fly rule is automatically enforced based on the situation. It is still a judgment call by the umpire.
Another point of confusion (and this is confusing) has to do with the ball being fair or foul. For example, you could have the umpire call out "Infield fly, batter is out.", and then not have the batter be out. How can that happen? The infield fly rule only applies to a fair ball. So let's say a pop up is hit down the third base line. The umpire makes his call and then the ball drifts into foul territory. Whether the ball is caught or not, the infield fly rule no longer applies. So, if the ball is dropped by the third baseman in foul territory, it's simply a foul ball and the batter is still up. In this case, umpires are taught to yell, "Infield fly if fair". The umpire may mistakenly make the first call and just because he called the batter out, doesn't mean he is out in this situation. The correct call is simply a foul ball.
A similar situation can happen if the ball hits in fair territory (before the bases) and then rolls foul (before being touched by a defensive player). You might see this on a pop-up to the catcher or pitcher. The ball is missed and the backspin takes the ball foul after it is missed. In this case just like the last, the infield fly rule doesn't apply and it's a foul ball.
To keep the same line of thought going, let's say a popup is hit to first base, the umpire yells "Infield fly, batter is out.". The first baseman misses the ball and it hits in fair territory and rolls into foul territory after going past first base. Since this would normally be a hit, the infield fly rule does apply and the batter is out.
Nope, there's more. Where the ball is caught can be another point of confusion. Let's say your second baseman is playing on the dirt and takes a few steps back onto the outfield grass to catch the popup and you hear the umpire call the infield fly rule. You think the player is on the outfield grass, so the call shouldn't be made. Again, the rule is written to give the umpire the ability to make the call based on his judgment. If you think about it, it really makes sense. In this situation can the umpire really determine whether the ball is going to land in the grass or the dirt? What if the grass is really close to the infield on this particular field? Or how would he make the call on an all dirt field? What he can do is judge whether the second baseman is in a position to make an ordinary catch and to visually determine and make a judgment as to the depth of the fielder. He can then determine to call the infield fly rule even if it ends up being caught in the outfield grass.
What if in that same situation the right fielder calls off the second baseman and makes the catch? The answer is that the infield fly rule would still be in effect. The rule is based on the judgment of the umpire while the ball is in the air. If the right fielder calls off the second baseman he is still catching what the umpire had determined to be an infield fly.
The other runners can try to advance at their own risk as they would on any other fly ball. If the ball is caught, the runners must tag up before advancing. If the ball is not caught, there is no need to tag up. The difference for the runner is that since the batter is out, there is no longer a force play and the runner doesn't need to advance, even if the ball is not caught.
The infield fly rule doesn't apply to line drives or a bunted ball. There's an additional rule that applies to an intentionally dropped ball including line drives and fly balls when the infield fly rule is not called. This rule states that the batter is out if a defensive player intentionally drops the ball with the intention of gaining a defensive advantage by not catching it. An example is there's a runner on first and a line drive is hit to the shortstop close to second base. The shortstop drops the ball in order to try and turn a double play. This rule differs from the infield fly rule in a number of ways:
An important part of this rule to understand is that the rule doesn't apply if the infielder permits the ball to drop untouched to the ground. So, if the shortstop, in the example above let's the ball hit the ground before fielding it (even though he could have caught it in the air), the rule doesn't apply.