Well I am still currently learning, but it is basically cutting the blood flow to brain (not only compressing jugular vein but compressing carotid artery so you pass out fast and peacefully if you do it correctly). If you only compress jugular veins as far as I know it would be painful and it would take longer because it is the vein that takes blood without oxygen from brain back to body (basically you would have carotid arteries working and supplying oxygen to brain so it takes slower and is painful).
There are two "sweet spots" (as they call) on your neck, you need to find the pulse. You can try compressing those spots with your fingers, if you do it correctly, you will feel some tingle in you feet, arms, or lips, I think this may depend on the person, but like in general if you feel like you are going to pass out, congratulations you found the spot.
So in theory, compressing this area will make you pass out in seconds, and your brain will be dead after sometime and you would be dead probably after 30 minutes passed (you need to make sure that you are not found before or you can turn into a vegetable).
The reasons it is unreliable they say are:
1- It is not easy to find the spots, compress them fully while making sure that the tourniquet (or what you are using, belt, ratchet belt preferably, I will explain in a second) doesn't and won't move when you are unconscious.
2- There are only a few footages, one that is easily found is a YouTuber's suicide, in the video he (tries, let's say) compresses his carotid arteries and jugular veins, but he tightens the ratchet too much and ends up compressing his windpipe as well (which causes SI to show up, that's probably why he suffered through the process) and he was making weird noises because he wasn't able to breathe properly. But I believe if done correctly, it would be a smooth ride, you would pass out fast and die as the time passes.
3- Failing is so easy in this method. In a usual good scenerio, you would pass out and wake up again right after. In the worst scenerio, you would get brain damage and you would have to live rest of your life as a vegetable, or you would die consciously while suffering in pain and that is not we want.
So in short, you need to know much about it both in theory and in practice, to achieve a peaceful death. And it seems that it is quite hard to do, but in my case I wouldn't choose it if I had some other option.
And how safe is practicing? Well, it's not really safe, I mean I didn't die from it (yet but I am currently determined to ctb with it), but I can imagine someone practicing and dying without intending it.
And that I guess all I know, sorry for the long-ass paragraph. You can check resources in this site for night night method, I will link it here:
https://sanctioned-suicide.net/threads/the-night-night-method-mega-thread.6834 I hope it helps
