There was a common misconception stemming way back to the original carbineer correspondent that counterattack when it triggered, made your next attack unstoppable (never missed) but that was put to bed years ago, it's simply the same as block (stops the incoming attack), but with a damage return as well when it fires. No idea on the math behind how much damage though, it's not a lot.
The attack you would have been seeing is the outgoing damage return, we don't get to see that here, it's not in the combat log.