This complex device, also called a VCU, relies on the characteristics of a special fluid inside it. It can act as a differential, as a means to restrict wheelspin or both. It consists of a small sealed canister filled with silicon fluid. Inside are two sets of slotted metal plates - one connected to a front shaft, the other to a rear shaft.
When there is a significant speed difference between the shafts, the metal plates spin in relationship to each other, heating up the silicon fluid which becomes thicker (more viscous). The thicker fluid slows down the metal plates, limiting the speed difference between the shafts, and thereby the slippage right-to-left or front-to-rear.
In effect, a VCU "locks" the shafts together (although not mechanically, as in a locking differential). When there is little or no speed difference between the shafts, the viscous coupling does no work. Range Rover, as an example, uses a VCU as a slip-limiting device in conjunction with its center differential. Lexus RX300 uses a VCU in place of a center differential.