Your Ambush, Garrote, and Cheap Shot abilities reveal a flaw in your target's defenses, causing all your attacks to bypass 100% of that enemy's armor for 10 sec.
Find Weakness is a passive specialization ability for Subtlety rogues, available at level 20. It causes you to ignore 100% of an enemy's armor for 10 seconds after using Ambush, Garrote, or Cheap Shot.
Patch 5.3.0 (2013-05-21): Now bypasses 100% of armor against non-player targets, and bypasses 50% of armor when used against other players (was 70% armor bypass for all targets).
Patch 5.0.4 (2012-08-28): Now a baseline passive ability for Subtlety rogues.
Patch 4.0.6 (2011-02-08): Now grants 35/70% armor reduction, up from 25/50%.
Patch 4.0.1 (2010-10-12): Now Subtlety talent that bypasses armor (was Assassination talent that increased offensive ability damage by 2/4/6%).