ORIGINAL: adam-69
They are called 'centrifugal clutches' because as the engine revs up, the centrifugal force exerted on the shoes increases and so they move out and engage with the bell. As the engine rpm drops the springs pull the shoes back in.
P.S. welcome to rcu!
What he said, its just that simple. There a few different types of designs but they are all centrifugal.