You can make damaging strikes against unprepared opponents.Prerequisites:
If you can catch an opponent when they are unable to effectively defend themselves from your attack, you can strike a vital spot for extra damage. Your attack deals extra damage anytime your target would be denied their Dexterity bonus to AC (whether the target actually has a Dexterity bonus or not), or when you flank your target. This extra damage is +1d6. Should you score a critical hit with the sneak attack, this extra damage is not multiplied. Ranged attacks can count as sneak attacks only if the target is within 30 feet. With a weapon that deals nonlethal damage (like a sap, whip, or an unarmed strike), you can make a sneak attack that deals nonlethal damage instead of lethal damage. You cannot use a weapon that deals lethal damage to deal nonlethal damage in a sneak attack, not even with the usual –4 penalty. You must be able to see the target well enough to pick out a vital spot and must be able to reach such a spot. You cannot sneak attack while striking a creature with concealment.Special:
You can take this feat multiples times; each time increasing the extra damage by +1D6. The extra damage dealt by the sneak attack cannot exceed 1D6 per two character levels. For example a 4th level character could sneak attack for +2D6 damage but not any more than that.