My understanding is that those convulsions are possible --or at least significant twitching as the electrical activity in your brain fades. That's why all the resources I saw recommend being sitting up, slightly reclined in a comfortable chair, so it's less likely for you to scrub the bag off your head, as you might if you were lying down and your head lolled about on a pillow.
I also heard it recommended to duct tape the gas cylinder to the leg of your chair, so you wouldn't knock it over. I effectively did that when I made my attempts: I used webbing lash straps to strap the tank to the chair.
I have long hair that I keep in a queue, with an elastic at my nape. I tucked the hose through the elastic, which seemed to work quite well. Though I didn't get far enough to have twitching/convulsions, it seemed very secure. I have also read the recommendation to tape the hose to the inside of the bag. What's more, run the hose up behind you, over your shoulder and into the bag at your nape; that keeps it more clear of your arms, in case they flop about.
I believe much of this is covered in more detail in the Inert Gas megathread.