The viscous differential is pretty cool, I have read about this before and was reminded.
It uses a non-newtonian fluid (like ooblek, but obviously not that one 
). Two plates are spinning. As long as they are spinning at about the same speed the fluid is… fluid… and its a slippy differential. If the two plates are spinning at different speeds the fluid becomes much more viscous locking the differential (or mostly locking it I suppose).
I’d imagine that viscosity switch just wouldn’t work at really low speeds though. Inching over some giant boulder or whatever.
But it sounds like that is now being replaced by computer controlled clutch mechanisms or by fancy gear based mechanisms in some cars.
