Rule 2-14-2 p. 26 Rule Book: A scrimmage-kick formation is a formation with at least one player 7 yards or more behind the neutral zone and in position to receive the long snap. No player may be in position to receive a hand to hand snap between the snapper's legs.
Play 9.4.6 p. 69 Case Book: Situation A: From the scrimmage kick formation, A1 snaps the ball to back A2 who is 3 yards behind the line and offset from the snapper by 1 yard. A2 runs for a 10-yard gain. Immediaely after the snap started, B7 charges: (a) directly into the snapper; or (b) into the gap between the snapper and the adjacent A player making simultaneous contact with both the snapper and the other Team A player. The snapper had not had the opportunity to defend himself and was displaced by B7's charge. Ruling: In (a) B7 has roughed the snapper. If accepted, it is a 15 yard penalty and an automatic first down. There is no requirement that the ball be kicked or that a deep back receive the snap. In (b) there is no foul. The snapper's protection does not include simultaneous contact with another A player, nor does it take away the "center-guard" gap from B. The roughing prohibition is only for a direct charge into the snapper.
Rule 9-4-6 p. 64 Rule Book: Roughing the snapper. A defensive player shall not charge directly into the snapper when the offensive team is in a scrimmage-kick formation. Penalty: 15 yards from the previous spot and an automatic first down.
Rule 2-30-14, p. 32 Rule Book: A snapper is the player who is facing his opponent's goal line with his shoulders approximately parallel thereto and who snaps the ball. In a scrimmage kick formation, the snapper remains a snapper until he has had a reasonable opportunity to regain his balance and protect himself or until he blocks or moves to otherwise participate in the play.