A suit of durable armor gives added protection by absorbing some of the force from attacks. However, in doing so, the armor takes damage.
The first time you take damage during an encounter, reduce that damage by an amount equal to the armor’s durability rating (for instance, 2 for ring mail). The armor then loses this property until you repair it during a short rest or an extended rest.
Durable armor provides greater damage absorption when it is enchanted. Add the enhancement bonus of the armor to the durability rating of light armor, and add twice the enhancement bonus to the durability rating of heavy armor.
Published in Mordenkainen's Magnificent Emporium, page(s) 8.