Article 4. Neutral Zone Infraction
It is a Neutral Zone Infraction when:
(a) a defender moves beyond the neutral zone prior to the snap and is parallel to or beyond an offensive lineman, with an unimpeded path to the quarterback or kicker, even though no contact is made by a blocker; officials are to blow their whistles immediately
(b) a defender enters the neutral zone prior to the snap, causing the offensive player(s)
in close proximity (including a quarterback who is under center) to react (move) immediately to protect himself (themselves) against impending contact; officials are to blow their whistles immediately. If there is no immediate reaction by the offensive player(s) in close proximity, and the defensive player returns to a legal position prior to the snap without contacting an opponent, there is no foul. A flexed or split receiver is considered to be in close proximity if he is lined up on the side of the ball on which the violation occurs; other offensive players are considered to be in close proximity if they are within two-and-one-half positions of the defender who enters the neutral zone.
If the defender is directly over the center, a quarterback under center, the center, and the guards and tackles on both sides of the center are considered to be within close proximity; if the defender is in a gap, the two offensive players on either side of the gap are considered to be within close proximity (including a quarterback under center, if applicable)
(c) a player, after he has received a warning, enters the neutral zone. It is a foul, even if he returns to a legal position prior to the snap without contacting an opponent or causing a reaction (movement) by an offensive player in close proximity.
https://operations.nfl.com/the-rules/2018-nfl-rulebook/