Pretty sure the intention is just if you take a hit that is for more than the current temp health, it should cause trauma on the excess. Right now it blocks any trauma from that hit.
I've got video proof that they are not stumbling even under extreme pressure on solo veteran and it needs to be fixed. That's 5 Barret 50 cal shots into it's back with no stumble. But then you see me Tec9 the armor off another and it stumbles within moments. Here's another example in a later level of 6 Barret 50 cal shots, with 4 straight to the weakpoint not stumbling a Crusher.
At first I was like "pfft, I stumble them all the time" but then I double checked just to be sure (as I try to do) and it turned out all the stumbles I was remembering was the forced stumble from breaking their armor.
/u/TRS_TheGentlemanSQ I just scanned through almost 2 hours of solo veteran gameplay (2 links to different sections listed above) and did not see me stumble a single tallboy, bruiser, or crusher with the Barr...
I'll dig into the stumbles tomorrow, but tonight I played through act 1 and I was seeing them though I was a bat Holly and we had a shotgunner with a stumble attachment.