ATB is stands short for Auto Torque Biasing.

This is a term used with Limited Slip Differentials.

A ATB LSD 'feels' how much grip a wheel loses and transfers some of the power to the other wheel.

In the picture above you can see a cut-out of a ATB LSD.

Here you can see that, in contrast to a clutchtype LSD, a ATB (sometimes also called a helical LSD) uses several helical gears.

Under normal conditions, when both wheels have the same amount of grip a ATB LSD will behave as a normal open differential (Which 90% of the productioncars use).

This enables the wheels to turn independently from each other in turns, but still be powered by the engine.

When one wheel loses grip, the difference in torque will make the gears bind together.

The bigger the difference, the more the gears will bind together.

The design of the gears determines how much torque will be transffered to the other wheel.

For instance: When a ATB LSD is designed with a bias ratio of 2:1, than the LSD can transfer up to twice the amount of torque to the wheel that does have grip!

And that is what we want of course, the power there where we can use it.

In this situation, a ATB LSD is superior to a clutchtype LSD, because it transfers the torque to the stable wheel  before the actual slipping occurs!  

And with a clutchtype LSD, the plates can slip themselves before they actually grip together (Think of the clutch between engine and gearbox, that slips slightly before it locks together aswell).

That causes a slight delay between slipping and the LSD locking, however marginal, it is not considered optimal when things happen fast.

Is a ATB LSD perfect than?

Nope, unfortunatly a ATB LSD has it's drawbacks aswell.

When one wheel suddenly loses grip completly (For instance when one wheel comes off the ground), Than the Bias ratio determines how much torque is transfered to the other wheel. And sadly, 2 x 0 = 0!

That means that there is no power going to the other wheel and the LSD still behaves like a normal open diff!

But there is a solution for this problem, although it is a bit weird and it is the complete opposite of what most people would do: Hit the brakes!

By braking, you apply torque to the wheel and twice that torque can by transfered to the other wheel again.

ATB LSD's are sometimes also called a Torsen differential.

I don't think this is quite right. Torsen is a registered trademark of Zetex Torsen inc. So In my opinion, it's a brandname and not the right name for this type of diffs. There are more manufacturers that make these LSD's