Severing Strike (3.5e Feat)
From D&D Wiki
Severing Strike [Fighter]
You learn how to disarm an opponent. Or disleg him.
Benefit: Choose one slashing weapon for which you have taken the Improved Critical feat. When you score a critical hit on an opponent, or beat his AC by 10 or more, and deal damage equal to or greater than his Con, you may choose to sever one limb. When you sever a limb, you may choose to do so without depleting your opponent's hit points
Normal: Normally, a critical hit just deals double damage.
Special: Some creatures have no limbs, and can't be affected by this feat. A fighter may take this feat as one of his bonus feats.