All cars with two driven wheels have a Diff...
This is just a limited slip differential. Hence it locks at certain amount of slip.
IE you go round a corner tight and plant the accelerator. The out side wheel is spinning faster than the inside front wheel. The engine naturally through a standard diff sends power to the outside wheel (As it is spinning faster) which is where you don't need the power. You need it on the inside driven wheel. The outside one needs as much grip as possible as the weight from the car and turn is being force upon it.
But what happens is a standard diff sends the torque to the outside wheel meaning the inside wheel doesn't get much and thus causes the outside wheel to spin. This means in a corner you run wide or cause a lot of tyre smoke.
A Limited Slip Diff means that it can't do that. It locks at a certain point (well transmits the torque to the inner wheel) allowing the car to essentially grip more power down.
Its mechanical advantage which really every car should have. They don't even cost that more in the first place and make huge differences.
But alas car companies are forever trying to cut costs somewhere.
Look up Limited Slip diff on you tube and you should get decent videos on how they operate.
Alfa 166 2.0 Twinnie - LPG and a few trick performance parts