It's really impossible to know that since the people who are successful (and theoretically might or might not feel pain while doing their method) are dead. No way to ask.
But the idea is you take benzo's and/or painkillers, drink a bit of alcohol while you wait for the coals to get to the right level, bring it into the tent (or car or whatever) and you go to sleep and don't wake up. I know it can cause a nasty headache if you wake up and you get the feeling you need to get out to live and exit the tent/car. That's why you want to make sure you pass out. I know of 2 people since joining that have used this method (one I know for sure was successful due to seeing their obit) and haven't come back so I assume they were both successful.