There are clutch shoes in the coupling that are kept inside with pull springs. At idle speed they do not make contact with the bus that is connected to the rear wheel, your engine can now run freely. If you give gas, they are thrown out against the clutch housing (that bus) and force is applied to the rear wheel.
But because the force with which the shoes on the clutch housing around it (and therefore the resistance that that engine gets) depends on the speed, the engine will thus at standstill at a certain speed during acceleration.
You can adjust this speed with the strong springs. After a few seconds pulling up, the shoes are fully pressed against the bus and the vario takes over the acceleration, through you a transition of the speed, usually from low to high / except then of course when your scooter is so well adjusted that the clutch speed exactly equal to that of the vario. then you do not hear the transition from the coupling; he then immediately moves from 0km / h to the top speed.