When your opponent is a attacking you, you can use cards from your hand to guard, the guarding unit's shield adds up to the victim's power, and the goal is to beat the attacker's power with your shields. Most units have 5000 shield, but trigger units (except draw triggers) have 10000 shield, and grade 2 units can guard from the front row of the rear guard, and action known as intercepting, wither the guard was successful or not, all units that took part in the defense go to the drop zone or discard pile. If defending against the vanguard, possible drive triggers can break the shield, also bear in mind that you can only call units of equal or lesser grade to your vanguard.