Your first attack deals a critical wound, so you follow the attack with another strike.

Encounter   ✦     Martial, Weapon
Minor Action      Melee weapon

Requirement: You must be wielding a dagger and must have scored a critical hit with a dagger against an enemy during this turn.

Target: The enemy you critically hit

Attack: Dexterity vs. AC

Hit: 3[W] + Dexterity modifier damage.

Update (10/4/2011)
Updated in Class Compendium.

Published in Player's Handbook, page(s) 127, Class Compendium.