Usually the waitlist starts moving when candidates deny the acceptance--and that usually happens. If there 30 slots and 30 acceptances have been sent out, that doesn't mean all 30 will be filled, as not all will accept. So if you're far up the waitlist, you have a good chance at getting in. At the very bottom, not so much. But I'm sure there's a scenario that's happened at a school where the waitlist didn't move at all because everyone accepted. As for the ones that don't rank their waitlist, I don't know how it works. First come, first served?