you can think of a centrifugal clutch as an automatic clutch (it basically is) if you give no throttle it stays unenegaged, but as soon as you give it some gas the engine revs up and the palls in the centrifugal clutch let out and grab the bell, completing the linkage from engine to wheel.
if the wheel is spinning (I.E. you are rolling) and you let off the throttle the centrifugal clutch automatically lets go, and you begin coasting.