The infield fly rule applies only when there are fewer than two outs, and there is a force play at third (runners on first and second base, or bases loaded). In these situations, if a fair fly ball is hit that, in the umpire's judgment, is catchable by an infielder with ordinary effort, the batter is out regardless of whether the ball is actually caught in flight. The rule states that the umpire is supposed to announce, "Infield fly, if fair." If the ball will be almost certainly fair, the umpire will likely yell, "Infield fly, batter's out!" or just "Batter's out!" Umpires also typically raise one arm straight up to signal to everyone that the rule is in effect.
Any fair fly ball that could have been caught by an infielder with ordinary effort is covered by the rule regardless of where the ball is caught. The ball need not be caught by an infielder, nor must it be caught in the infield. For example, if an infielder retreats to the outfield in an effort to catch a fly ball with ordinary effort, the Infield Fly Rule would be invoked, even if an outfielder ultimately caught the ball, and even if no infielder attempted to make a play on the ball. Similarly, a fly ball within the infield that could have been caught by an infielder with ordinary effort, but is caught by an outfielder, would also be covered by the rule.
On a caught infield fly, a runner must tag up (i.e., retouch, at or after the time the fly ball is first touched by a fielder, the base the runner held at the time of pitch) in order to be eligible to advance, as on any catch. If the infield fly falls to fair ground untouched, or is touched and dropped, runners need not tag up. In either case, since the batter is out, the force play on other runners is removed.
The infield fly rule cannot be invoked on line drives or bunts; also, the infield fly rule is not intended to cover all situations where the defense may wish to allow a fly ball to drop uncaught. For example, with just a runner on first, an alert infielder might intentionally allow a popup drop to the ground and get the force at second, if it happens that the runner on first is faster afoot than the batter-runner is, or if the batter is loafing on his way to first base. This is only legal if the fielder lets the ball hit the ground untouched, which carries some risk to the fielder as it might bounce away from him. However, in all situations where the infield fly rule does not apply, a different rule (6.05l) prevents fielders from touching a catchable ball and dropping it intentionally in an attempt to turn a double or triple play.