On my Radster, I don't think it can be switched while in motion. Class 1 disables the throttle, and class 2 works like you'd expect (throttle to 20 mph). In class 3, throttle will still take you to 20 mph but no more; to go any faster you have to pedal, and pedal assist cuts out at 28 mph. The Radster also has a fourth setting which tops out at 25 mph for NYC riders.