I experienced a few times, if the player is fully surrounded (or backing to the wall, then surrounded), Bardin will let hits in during Impenetrable until he’s downed.
The tooltip says: Bardin taunts all nearby man-sized enemies and can block any attack for the duration. Enemy push is an attack as well as hits (?). If I understand well, Flamethowers’ flame, Rattlig Gunners’ fire and Ungor archers’ arrows (except shields), Trolls’ bile and Halescourge’s projectiles cannot be blocked even during Impenetrable.
It happened 3 times, first it was a chaos patrol (can push), the 2 others were simple vermin hordes. I used dual hammer all 3 times, holding the right mouse (block) button, and I didn’t use push. My mouse button is not faulty, I tested it. I just stood there, waiting the others to kill the taunted horde, held block, then one hit got in, broken block sound and animation, another hit got in, then again broken block sound + animation, and so on till they downed me.
Other: is Impenetrable supposed to taunt Chaos Warriors? Because sometimes it does for the whole duration, no matter anyone else hit or pushed them, sometimes they seek another player during the taunt. Other tooltips say Chaos Warriors do not count as man-sized enemies.