When runners are on base with less than two outs and a ball is hit in the air that does not go past the infield, the batter is automatically declared out thus preventing the fielders from purposely dropping the fly ball in an attempt to force out runners.
A rule which declares the batter out because, in the judgement of the umpire, a fair fly ball hit within the infield can be caught by an infielder facing the infield when at least first and second bases are occupied and there is less than two out. For this purpose, an outfielder that is stationed in or near the infield is also considered an infielder. The umpire signals that the rule is in effect by calling "infield fly" and raising a clenched fist overhead. The ball does not have to be caught. The batter is out but the ball is still live and all baserunners may try to advance at their own peril. The rule was instituted so that an infielder does not deliberately drop the ball so that a double-play can be set up.
On the infield fly rule the umpire is to rule whether the ball could ordinarily have been handled by an infielder not by some arbitrary limitation such as the grass, or the base lines. The umpire's judgment must govern, and the decision should be made immediately. When an infield fly rule is called, runners may advance at their own risk. If on an infield fly rule, the infielder intentionally drops a fair ball, the ball remains in play.