The infield fly is in effect by rule when the ball leaves the bat. There can be no force play because the batter is out immediately. Eliminating the force is the reason the I.F. was invented. Also by rule, the umpire is supposed to call it as soon as he/she recognizes it whether during the ball's flight or after the fact. Since it is a fly ball that can be caught, runners must tag up and not leave the bag before the ball is touched (not necessarily caught). Leaving early is appealable by the defense. If the I.F. becomes a foul ball, the ball becomes dead as with any foul ball and the I.F. is negated. If asked, way too many umpires will incorrectly tell you that the batter is not out until it is determined that the ball is not foul. Here is a great study in why it is important that everyone (especially umpires) understand the details of the infield fly rule.