Sense Weakness

(Draconomicon, page 106, DnD 3.5)

[General]

You can take advantage of subtle weaknesses in your opponents' defenses.

Prerequisite

Combat Expertise, Weapon Focus, INT 13

Benefit

Whenever you attack with a weapon with which you have selected the Weapon Focus feat, you may ignore up to 5 points of the target's damage reduction (regardless of the material or enhancement bonus of your weapon) or hardness. This benefit can't reduce the effective damage reduction or hardness of a target to less than 0.