From the Rulebook:
A hit to armor from a weapon, Magic Ball, etc. with the Armor Breaking ability will immediately remove all Armor Points from the location struck if the location struck currently has three or less Armor Points.
If the armor currently has four or more Armor Points then the armor has one Armor Point removed as per normal.
Example: Armor with 4 points is struck by a weapon with Armor Breaking. The armor loses an Armor Point at the location struck. The armor is struck again in the same location which now has 3 points. The Armor Points at that location are reduced to zero and the wearer is unharmed unless otherwise noted.
Denoted by announcing “Breaking”.