[cwmcobra]

The friction modifier is actually used in limited slip differentials, not locking differentials. Lockers have a mechanical locking mechanism that solidly locks the axles so that both always turn until they are unlocked. The limited slip differential has a disc pack that acts like a clutch. It never totally locks up, but does transfer some of the torque to the non-turning axle when the driven axle (wheel) starts to slip. The friction modifier is to keep the disc pack from partially locking when you don't want it to, like when you go around a corner. You might have experienced an occasional "chirp" when turning corners with a LSD. This is when the discs start to lock due to friction among them. The friction modifier is intended to help this situation. It doesn't totally eliminate it, but it helps.

My company makes these differentials, so that's my source, but its a layman's view, since I'm not an engineer on these products.

Chuck