Normally, clutch plates and discs are alternated – plate-disc-plate-disc, for example – but if two friction discs are sandwiched together, that reduces the number of active plates in the diff, reducing the locking effect.
Put simply, preloading provides a base level of locking across the diff, so that it is never running completely open. This is generally referred to as ’breakaway torque’, and can be measured as the force needed to rotate one wheel while the other is stationary. Preload is generally achieved using Belville washers on the external face of the friction plates. You can adjust it by changing the thickness of Belivlle washers or discs.
The ramps govern how aggressive a differential is in its locking action. Traditionally LSDs have been referred to as one, 1.5 and two-way units, which relate to the action of the ramps. In a one-way unit, locking is only achieved under acceleration; in 1.5-way the angle of the acceleration and deceleration ramps is different, and in two-way diff the ramp angle is the same in both phases. For the vast majority of modern sports cars, the most desirable will be 1.5-way.
Butcher, L 2019. ’Rally race technology’ annual report 2019., pp. 54-60.