The Bratex helical LSD's are torque biasing, that is they distribute torque between the wheels, loading more torque towards the wheel with the best traction thus improving performance and driveabitily. Especially in low grip situations.
The helical gearing manages the distribution of friction within the LSD. When torque is applied to the helical gear system it creates force that pushes the gear into the differential body, thus creating friction. Increases of torque, increases the thrust force directly proprotional to the amount of torque. This allows the Bratex LSD to cope with considerable traction imbalance even under heavy power conditions, as would be the case on exiting a powered up corner. But at low torque conditions the LSD can differentiate quietly and smoothly, thus making the differential very suitable for normal road driving. The Bratex LSD uses the popular parallel axis helical geared configuration optimised for even load distribution.
They are designed with a torque bias of 5:1 this means that if  one wheel loses traction then the wheel with the most grip can receive five times as much torque as the wheel that is slipping. However, five times zero is still zero, so all these helical LSD's work best when both wheels are 'planted' and some traction is still available on the slipping wheel. In practice this is rarely a problem as there is always some 'drag' on the slipping wheel, however some drivers expertly use left foot braking to partially lock the spinning wheel.

Vehicle affectation:

BMW E21 315, 316, 318

BMW E21 320i

BMW E21 323i (without OE LSD)

BMW E30 318i and 318is

BMW E36 316i, 318ti

BMW E36 318is, 320i

BMW E53 X5 4.4i (auto front differential)

BMW E53 X5 4.6is (front differential)

BMW E83 X3 3.0d (front axle)

BMW E83 X3 3.0sd (front axle)

BMW Z3 Z3 1.9 (manual)

BMW Z3 Z3 2.0 (manual)