heres a bit of an explanation-
http://www.offroaders.com/tech/limited-slip-lockers-differentials.htm
The main purpose of the differential is to allow each half of the axle (each tire) to spin at different speeds, while supplying an equal amount of force to each wheel in that axle. The need for the wheels to rotate at different speeds is especially apparent when turning corners. When cornering the inner wheel travels a shorter distance than the outer wheel. With an open differential they both propel the vehicle forward with equal force, so long as both wheels remain in contact with the road and have traction. However if one wheel slips, for instance on ice, more torque is sent to the wheel that spins. If that slipping wheel completely looses traction, all power will be sent to that wheel and you have no forward momentum. When offroad, this is where the common open differential fails to remain effective. When offroad there are many situations where a wheel will spin free. In most stock 4x4 vehicles the common Open Differential can be found in both the front and rear axles. When a wheel in the front AND a wheel in the back are allowed to spin free due to the Open Differentials, that 4x4 is essentially a 2 wheel drive vehicle. One front wheel, and one back wheel.
so pretty much.. if both tires in the front have good traction, you wont get much slippage- same for the rear. but if one tire on each axle looses traction, that's where you get the 2 wheels spinning-