CENTRE DIFF
Four-wheel-drive vehicles not only have a differential at the front and rear, they also have a centre diff too. The front and rear diffs work in the conventional way, distributing torque across the axle to a pair of wheels, however a 4x4 car needs a centre diff to control the distribution of torque to the front and rear too.
For example, the Ferguson system as used in the Cosworth 4x4s uses viscous front and rear diffs and a centre diff with a viscous coupling to split the power 34:66 front-to-rear. This is needed to allow the car to turn-in better. Without this split or with a 50:50 split the car would tend to understeer because although both the outside wheels will have further distance to travel then their opposite number, the front outside wheel will have less distance to travel than the rear outside wheel. The front wheel will steer so alters the path it takes, whereas the rear wheel is fixed and so has further to travel.
To allow this torque split, the Ferguson system uses a centre diff with epicyclic gears. It is a complicated bit of kit, but basically as the whole unit turns a set of epicyclic planet gears within an annulus ring gear also turn. The epicyclic planet gears are in contact with both the annulus ring gear and a sun gear. The resulting reduction in gearing allows the force going to the front to rotate more slowly than the force going to the rear. This reduction in gearing equates to a 34:66 split front-torear.
A viscous coupling is added to the centre diff to promote equal torque distribution in the same way it would in either a front or a rear diff. This is why when you boot it on a loose surface such as gravel all four wheels will spin at the same rate, but when you turn a corner on Tarmac all four wheels will rotate at different speeds to allow the car to turn without understeer or oversteer.