You have learned to attack with the weapon an opponent does not suspect.
Benefit: You can attempt to gain an advantage by feinting with a primary weapon, then attacking with a secondary. When you are wielding two weapons, as a standard action you may make a single attack with a light weapon in your off-hand. Rather than a standard attack roll, make a Bluff check. If your skill check exceeds your target’s AC, your attack hits. (Deal damage as an off-hand attack, which normally means you add only half your Strength bonus to the damage of this attack.) If your skill check also exceeds a DC of 10 + the target’s base attack bonus + the target’s Wisdom modifier (or 10 + your target’s Sense Motive bonus, whichever is higher), your target is also denied its Dexterity bonus to AC against this attack.
Adventurer’s Handbook: Genius Guide Volume 1