Never seen / driven one, but my understanding is that they have two clutches. One is a conventional clutch, the other is a centrifugal clutch. So you don't need the normal clutch to pull away, this function being taken by the centrif clutch which comes in as the revs rise, thus also allowing progress in traffic just by controlling the engine revs. Once on the move, the normal clutch is used for gear changes.
Yet another nice design touch that was probably canned by the bean counters.