Infield Fly Rule
With runners on first and second (or bases loaded) and fewer than two outs, a catchable infield pop-up makes the batter automatically out.
The infield fly rule prevents the defense from intentionally dropping an easy pop-up to turn a cheap double play. With runners on first and second, or bases loaded, and fewer than two outs, the umpire calls the batter out on any fair pop-up an infielder can catch with ordinary effort, whether or not it is actually caught.
Because the batter is automatically out, the runners are no longer forced. They may advance at their own risk, but they don't have to, so the defense can't exploit a dropped ball to retire them on a force.
It only applies to pop-ups in the infield, not line drives or bunts, and only in those specific base-and-out situations.
When does the infield fly rule apply?
With fewer than two outs and runners on first and second or bases loaded, when an infielder can catch a fair pop-up with ordinary effort. The batter is out automatically and the runners are no longer forced.
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