Method works by replacing oxygen with inert gas such as nitrogen or argon.
No carbon dioxide is produced when you breath these in, so no feeling of suffocation.
You pass out from lack of oxygen and then your brain and body start to shut down from the same.
The lack of oxygen causes involuntary muscle movements while unconscious, which are usually mild, but can sometimes cause the bag to slip off one's head. Why some people strap themselves down.
It take a bit for lack of oxygen to kill you. If the bag comes off at some point, you'll start breathing again, but may have significant brain damage, leaving you a vegetable.