In football the neutral zone -- the area between the offensive and defensive lines -- is established when the ball is ready for play. At that point, neither team is supposed to enter the zone. The neutral zone only lasts until the ball is snapped, at which point the entire field of play becomes fair game, until the whistle blows and the ball is again marked ready for play.
Neutral Zone Infraction
A defensive player who enters the neutral zone will be called for a 5-yard penalty in certain cases. Under Rule 7, Section 4, Article 4(a) of the NFL rules, if a defensive player moves beyond or is parallel to an offensive lineman before the ball being snapped, and if the defender then has an вЂњunabatedвЂќ path to the quarterback or kicker, play is stopped immediately and the defense is called for the infraction. The defender is not given a chance to return to his side of the neutral zone. Pursuant to Article 4(b), if a defender enters the zone and causes an offensive player in вЂњclose proximityвЂќ to move, the defense is charged with an infraction. Players in close proximity include linemen within 2 1/2 positions of the defender. If the defender is over the center, for example, the five linemen from tackle-to-tackle, plus the quarterback, are considered in close proximity. If the defender is in the gap outside of an offensive tackle, only the tackle and guard closest to the defender are in close proximity, as well as any receiver lined up on the defender's side of the ball.
Neutral Zone Non-Call
A defensive player who steps into the neutral zone doesn't automatically draw a penalty. If a linebacker takes one stride beyond the point of the football, for example, then returns to his side of the ball before it's snapped, no penalty will be called, provided he neither makes contact with an offensive player nor causes an offensive player to move.
Offside and Encroachment
If a defender crosses the neutral zone and fails to return to his side of the zone before the ball is snapped he is guilty of a 5-yard offside penalty. This includes a player who lines up in the neutral zone. If a defender violates the neutral zone and makes contact with an offensive player before the snap, he is called for a 5-yard encroachment penalty.
An offensive player can only be called for a neutral zone penalty if he lines up offside. Once an offensive player other than the quarterback is set in his stance he may not move until the ball is snapped, except for a legal shift. Therefore, if a lineman takes his stance, then moves before the snap, he's called for a false start, even if he steps forward into the neutral zone. The idea is that offensive linemen may not try to trick defenders into violating the neutral zone. The quarterback, however, may try to draw defenders into the zone via a tricky snap count. For example, the quarterback may alter his cadence or yell a preliminary snap signal particularly loudly to fool the defense into believing the ball is about to be snapped, hoping they'll surge forward into the neutral zone and draw a penalty.