They work on Centrifugal energy...What you install is a plate on the outside of the pressure plate, that contains flyweights. When the clutch spins (hence centrifugal, the flyweights on the outside of the pressure plate apply pressure to the plate. Which squeezes the fibers and friction plates together, causing them not to slip.........Actually makes a clutch last alot longer!!! And thats why they are called lockups!!
Hope this helps...