[Christ]

Older Honda Civic WagoVan models with RT4WD had a viscous coupling unit between the rear axle and xfer case. Xfer always driven, but the viscous coupler was disengaged unless the speed differential was X-RPM between the input and ouput of the viscous coupler.

Literally, that coupler can be several thin plates meshed together longitudinally, which work on the boundary layer principle and the thermodynamic principle of heat expansion. They're basically soaked in 10W30 all the time, and as one side begins overspeeding the other side, the viscous fluid heats up, expanding, which increases the boundary layer effect of the oil on the plates, making a viscous bond between input and output until there is no more slippage, at which point, the oil cools down. The system wastes some energy as heat to keep transferring power, so it's not 100% efficient. More like 90%, I think. The efficiency and stall speed (the speed differential at which it begins transferring power due to heat generation) can be affected by the gap between plates, and the thickness of the oil.

This is not the only type of system, but it's a very rudimentary way to explain how the system works.