If you roll double sixes on a defense roll, you have made a critical defense. Critical Defense only occurs on natural double sixes, not by rolling above the attacker’s roll by a certain amount.
If you make a critical defense, you successfully throw off your opponent’s guard. The attacking opponent immediately “spends” one defense – essentially, your critical defense counts as an attack against the opponent that automatically misses.
Critical Defense can be improved with the Seize the Advantage Combat Technique, found under Combat Techniques.