Hits that deal zero damage are able to apply bleed. One on hand this allows tank attacks to deal decent horde damage, on the other it’s allowing you to kill bulwarks by slapping their shield a few times.
The issue is compounded by the current bug of bleed applying more stacks of bleed.
You wouldnt be able to kill an ogryn applying one bleed stack at a time otherwise because the damage would be much lower and the duration would be inly a few seconds instead of until it dies.
Bleed is actually terrible if it was working as intended.