You probe the defenses of your foe until you can land a blow that momentarily leaves it susceptible to other attacks.

Encounter   ✦     Martial, Weapon
Standard Action      Melee weapon

Target: One creature

Attack: Strength vs. AC

Hit: 2[W] + Strength modifier damage. Until the end of your next turn, you and your allies can score a critical hit against the target on a roll of 18–20.

Published in Player's Handbook, page(s) 148, Class Compendium.