When you strike an opponent’s vital areas, you draw on your ability to land crippling blows to make the most of your attack.
Prerequisite: Skirmish or sneak attack ability.
Benefit: When you score a critical hit against a target, you deal your skirmish or sneak attack damage in addition to the damage from your critical hit. Your critical multiplier applies only to your normal damage, not your skirmish or sneak attack damage. This benefit affects both melee and ranged attacks. Skirmish or sneak attack damage is added only once, even if you meet other conditions for adding damage, such as if you scored a critical while the enemy was flat-footed.
Source: Player’s Handbook II, page 83. Corrected for errata.