You have mastered the style of fighting with a halberd, and can use all parts of the weapon—blade, spike, hook, or butt—to strike devastating blows.
Prerequisites: Combat Reflexes, Two-Weapon Fighting, Weapon Focus (halberd).
Benefit: When you make a full attack with your halberd, you gain a +1 dodge bonus to your Armor Class as well as an additional attack with the weapon at a –5 penalty. This attack deals points of bludgeoning damage equal to 1d6 + ½ your Strength modifier.
Source: Complete Warrior, page 114.