It involves crossing the blades and locking the hilts, forming an X. One can "catch" the sword in the V of the two blades and effectively neutralize the blow. It is most often used by Rangers to fight close-combat with a swordsman. It is rendered completely ineffective when facing a person wielding an axe. During their "trip" to Celtica, Gilan teaches this defense to Horace and Will. Horace then uses it as a last ditch maneuver to kill Morgarath, which he does successfully.
Will turns Gilan's own lesson on the maneuver against him when he asks how it would work against an axeman. Gilan ends the conversation darkly by advising to jump off of a cliff he'd used as an example to convince Will the defense was necessary to learn.
The people known to use the Double Knife Defense: