The viscous coupling has a stack of very closely spaced plates in it. Half of them are connected to the transmission, and the other half to the rear end. They are alternating, so one is connected to the tranny, one to the diff, etc. Normally it's just front wheel drive. But when the front wheels start to slip, the fluid heats up and locks the plates together, transferring power to the rear wheels. Then the fluid will start to cool and power will be gradually removed from the rear wheels.