Stumbled onto this explanation of how a Torsen differential works. I'm still trying to wrap my head around it, but it basically uses the one-way nature oh how a worm gear can turn a regular gear but not the other way around (due to the high gear ratio involved).
So in this differential, the housing can turn both wheels simultaneously, but if one wheel slips and becomes free, it can't direct the torque to the free wheel:
Oh, that's a clever design. The explanation, even with all those graphics, is poor. The idea is that one wheel can go slower than the speed it would go if the axle was rigid, but it can't go faster.
With that design (which seems to be a Torsen Type A), the faster wheel does all the work of propelling the vehicle on turns, but the wheels lock together if one starts to slip. So it's for off-road vehicles, like the military HUMMV. (But not the civilian Hummer H1. That used a Dana differential more suited to on-road driving at higher speeds.)
There was a Torsen Type A in the B5 platform revision of the Volkswagen Passat 4motion. It was in the center. Front and rear were plain differentials, with the brakes used for left-right issues.
So in this differential, the housing can turn both wheels simultaneously, but if one wheel slips and becomes free, it can't direct the torque to the free wheel:
https://www.youtube.com/watch?v=JEiSTzK-A2A
Maybe someone has a better explanation?