The infield fly rule exists to stop fielders from deliberately letting a simple pop-up drop so they can turn an easy double or triple play. The hard part is knowing when the rule is available and when the umpire should call it.
“Ordinary effort” does not mean that the nearest infielder must stand still under the ball, nor does it depend on whether that particular player is usually a strong defender. The umpire considers the height, flight, wind, lighting, positioning, and time available to settle under the ball. The call can extend into shallow outfield when an infielder could handle the fly routinely, and it can be withheld on a difficult pop-up inside the dirt. What matters is the character of the opportunity, not the grass line or the eventual identity of the fielder who touches it.
An infield fly must be fair. When the ball is close to a line, an umpire commonly declares “infield fly, if fair.” If it settles foul untouched, is first touched over foul territory before passing the base, or otherwise becomes a foul ball under the ordinary fair-foul rules, the batter is not out on the infield-fly call. If it becomes fair, the batter is out even if nobody catches it. The declaration should be made while the ball is in flight when the umpire can judge ordinary effort, but the ball's final fair or foul status still controls.
Once the batter is out, runners are no longer forced to advance merely because the ball drops. They may remain on their bases. If the fly is caught, normal tag-up rules apply and a runner leaving early can be retired on appeal. If it is not caught, runners may advance without retouching, but they can still be tagged while off base. The defence cannot manufacture force outs by dropping the ball, yet it can record outs through ordinary tags or valid appeals. That distinction explains why runners often wait close to their bases until the catch question is resolved.
The call is a judgment made while the fly is developing, not a reward applied only after a fielder deliberately drops it. Umpires point upward and announce the infield fly loudly so runners and fielders know the batter is out. If an obvious qualifying fly is not announced, the rulebook allows the situation to be corrected when necessary to protect the runners, but crews aim to communicate before anyone chooses how to play the ball.
Because the rule is designed for exactly that situation. Once the umpire calls infield fly, the defence cannot gain the force-play advantage from choosing not to catch the ball.