For limited slip/posi rear end's with clutch pack's. Basiacally without the fric. modifier the clutch pack's make a bunch of annoying racket when turning, mainly because there causing friction against each other. Fric. Modifier is an additive that quiet's the chattering down, by allowing the clutches to slip a bit, is really all there is too it. It also increases the life of the clutches if moderate amount's of fric. modifier is used. Don't put too much though, or the clutches will end up slipping too much and you won't have much of a posi left.