As was already mentioned, you will lock the brakes and turn off a power wheelchair or ECV after getting on the lift. That will prevent the wheelchair or ECV from rolling.
On the buses with lifts, there is a small "lip" that comes up in front of you as the lift begins rising off the ground. That also prevents the wheelchair or ecv from mmoving. When you are getting off the bus, the lift is in the top position and the little lift is up. It will stay in the "up" position until you are totally lowered to the ground.
If you are planning to get out of the wheelchair/ecv and sit on a bus seat, let the driver know. He/She will still secure the wheelchair/ECV, but can do it in a faster way.
If you haven't already looked at the disABILITIES FAQs thread, one of the posts on that thread includes a lot of bus information.