Interceptor Missiles (ICM) are small missiles that can be fired at incoming torpedos, assault rockets and rocket battery barrages. Before the torpedo or rocket attack is resolved, the player controlling the target ship can declare that it is launching ICMs. The player must declare how many ICMs the ship is using. The attack's chance to hit is determined as if the attack was hitting a reflective hull or a masking screen, whichever the ship is using. The number of ICMs launched by the target ship is multiplied by the modifier on the Combat Table, and the result is subtracted from the attack's chance to hit.
EXAMPLE: A torpedo is launched at a ship with a reflective hull. The player that controls the target ship announces that it will fire two of its ICMs in defense. The torpedo normally has a 70% chance to hit, but each of the ICMs lowers this by 10%. The torpedo's final chance to hit is !70 - 20 =) 50%. A ship carries a limited number of ICMs. Whenever one is used. it must be checked off the ship's record sheet. ICMs can defend only the ship that launched them.
Source: Knight Hawks