Telling Blow( Player's Handbook II, p. 83)
When you strike an opponent's vital areas, you draw on your ability to land crippling blows to make the most of your attack.
Skirmish or sneak attack ability,
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.