One risk defending units have to avoid is going beyond what we call the threshold. When a defending unit comes forward flat and the attacking team is lying deep, it is important that the outer defenders do not go beyond the depth of the player carrying the ball. Rush defences are occasionally guilty of going too far forward on the outer edge, allowing the ball carrier to swerve in behind the next defender or turn him/her inward, allowing the next attacker the chance to receive a pass virtually unopposed. This situation also allows the ball carrier a clean gap to put in a grubber behind this over-anxious group of defenders.
This diagram shows a defensive line holding at a distance that prevents the ball carrier from getting behind:
If the defence comes up too fast, attackers can get behind or they have to stop dead to prevent going over the threshold. Both instances provide opportunities to the attacking team, so it is important that defenders be calculated and coordinated when coming forward, maintaining a pace that puts pressure on the attacking team but that doesn’t get them into trouble.