VISCOUS CENTER DIFFERENTIAL

Porsche has used a viscous coupler as the center differential in the AWD cars

A viscous coupling is filled with a silicone-type fluid. A series of plates rotate in the fluid; some of the plates are attached to an output shaft on the transmission, and some are attached to the front driveshaft. The viscous nature of the fluid causes the plates to move together; if the rear axles are turning faster than the front (eg. the tires are slipping), the friction between the plates causes the torque to be transferred to the front axles.