As I amass all the items I need for this and ctb gets closer to reality, I feel like I am missing something important and vital. This method seems too easy, too good to be true.
It is too easy, too good to be true, but probably not for any lack of the mechanism.
I have tried three times now with eb/N2. My failures have been entirely due to an overactive survival instinct: I pull the bag away just as I'm starting to black out.
Here's where I've had trouble: The recommended approach is to put the bag on your head like a shower cap, scrunch out all the air, and then allow it to inflate with inert gas (N2). Running the gas tube up your back was suggested, and it works well for me; I affix it at the back of my head with a hair elastic around my queue.
With the bag inflated with N2, you hyperventilate to clear your lungs of CO2, exhale deeply
and hold, pull the N2-filled bag down over your face and settle the elastic around your neck, and then take a deep breath of inert gas.
I'll bet that approach works very well. The trouble I have consistently had is that I'm
very anxious --I am killing myself, after all-- and I can't seem to exhale very deeply to begin with, nor can I breathe deeply once the bag is on my head. My adrenaline is singing, and I end up breathing very fast and shallow.
The result is that I don't pass out very quickly. I'd love to give you a specific amount of time it take before my hands start to tingle, my head gets swimmy, and my vision starts to grey, but stress-induced time dilation is pretty intense, and I haven't any idea how short or long a time it is before I pull the bag away.
The end result is that it takes longer than you might expect, and you have plenty of time for survival instinct to kick in and foil your plans.
As for convulsions/twitching, I have read that you don't want to lie down, because the more freedom you have to twitch, the more likely the bag is to be displaced. You want to be sitting up, so that if your head does move around at all, it doesn't have the entire force of your body behind it. Honestly, if you've built your eb properly, with good elastic and a cord lock, I don't think it'll displace easily. Mine is surprisingly secure.