When you make your armor take the brunt of damage dealt to you, the damage is negated but you must reduce the armor value of your armor or shield (your choice) by 1. The value is reduced each time you make this choice. If the reduction leaves the item with 0 armor it is destroyed.
The information on this site is based on the works of Sage LaTorra and Adam Koebel.
More information can be found at the Dungeon World Official Website.