Modern American football usage
Since at least 1923, the onside kick has been subject to additional constraints in most forms of American football, and the term is now something of a misnomer in American football. The receiving team has in general a presumption to a kicked ball, unless a player on the team touches the ball and muffs or fumbles it, upon which the ball becomes live and may be recovered and advanced by the kicking team. Otherwise, the restrictions that must be met in order for the ball to be recovered are:
* The kick must be a free kick (a kickoff, free kick after a safety, or the rare fair catch kick)
* The kick must cross the receiving team's restraining line (normally 10 yards in front of the kicking team's line)
* The kicking team may only recover and retain possession of the kicked ball, but not advance it
* The kicking team must not interfere with an attempt by a player of the receiving side to catch the ball on the fly [2]
"Onside kick" is now reserved in modern usage for a free kick intentionally attempted in such a manner as to maximize the possibility of recovery by the kicking team. Kicks not attempted in such a way that yet happen to be muffed or fumbled by the receiving team are not referred to as onside kicks.
Once the ball has hit the ground there is no chance of a catch and hence no possibility of interference. Thus the kicking team generally attempts to make the ball bounce early and be available around 10 yards in front of the spot of the kick. One technique, useful especially on a hard field such as one with an artificial surface, is to kick the ball in a way that it spins end-over-end very near the ground and makes a sudden bounce high in the air. The oblong shape of an American football can make it bounce off the ground and players in very unpredictable ways. This unpredictability has the additional benefit for the kicking team of increasing the probability that the receiving team will muff the kick.
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