It sits between the main side gears on an open diff, coupled around the pinion gear shaft, as the speeds between side gears change, the pinion twists the phantom grip so it exerts pressure on the side gears effectively locking or limiting 'slip'.
They dont fit in all box's, they relay on mechanical metal to metal friction so there is wear on the phantom grip (basically a metal slab with 2 holes passing though each side then cut in half), they dont always work a)well b)at all c)with the combination gearbox you are installing them into.
Some people love them because they work and are cheap, some people hate them..........most people who like them tend to have them in drag cars.