Hey there, it’s supposed to grant a 25% boost to all damage for 4 seconds after his shield (the physical one) is overloaded, giving you a chance to rage for a bit when your defense breaks. Maybe attack speed would’ve been better…? Anyway, we’ve been patching at this Gear for a while now, and it just keeps finding ways to not wanna be patched. 
I’m currently checking on the status of the Gear, but I think it’s actually working now, it’s just the item card display number that’s still being troublesome.
Sorry for the inconvenience, Boldur fans! We’ll get it wrangled shortly!
25% is quite a bit but the only possible use i can imagine out of it is intentionally letting the shield break and then throwing out a really strong runic axe or bashing them into a corner and hitting them a couple times.