This will only work with tempered glass. When you use it like a knuckle duster you will very likely be bruised without breaking the glass. The best technique would be punching with the radius of the carabiner (very much like hammering a door with your fist). The glass tends to take more than one punch before it breaks. You should wear gloves when you try. Punching with a pointy object (e. g. a nail) breaks the glass with less force.