Back Slice (technique)

source

master technique — attack

Make a melee weapon attack or unarmed strike as normal. If you hit and damage a creature, you permanently damage its armor—if it isn't natural armor, magic armor, or a shield. The target's AC is reduced by 1. If the target's AC is reduced to 10 (or 12 with a shield), its armor is destroyed.
     If you make this attack as part of a target strike, the target's AC is instead reduced by 2.