Telling Blow

 
Prerequisite(s): Skirmish or Sneak Attack class feature.
 
Benefit: When you score a critical hit against a target, 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.
 
Source: Players Handbook II