Fencewire is correct. The term threatened is used in the rule book. By rule, a maximum of 3 players can be threatened depending on where the defensive player is lined up. If he is lined up in a gap, then only the player to his right and left of the gap could be threatened. If he is lined up directly in front of a player, then the player he is lined up in front of, as well as the player to the right and left could be threatened. If very important to also realize that sometimes the defense will jump, but not enter the neutral zone, causing an offensive player to react. This would not be offside, but instead it is false start. Entering the neutral zone is the key.