Have been experimenting, this is my experience. The rope is about 10mm thick, similar to that used for climbing and wore a pullover with a high neck. The rope was secured to one of the bolts holding the water heater in the bathroom, which is rock solid, about 5 feet off the ground. I placed the slipknot under my left jaw fairly tightly but just pressing and sat in a kind of crouching position, so my backside was about 2 feet off the ground. When i started to allow gravity to apply pressure, more so on the right side of the neck,as the knot was on the left, my head was forced to tilt to the right. After about 10 or 15 seconds i knew that i was going to black out pretty soon if i didn't stop, was becoming faint and my head was beginning to throb. I imagine if i had consumed a decent amount of whisky over the evening and then taken a handful of pills, (say 50mg xanax and 100 mg of zopiclone, cruched up with a spoon in some jam) that would have been that. Also some antiemetics during the evening to try to avoid all that coming back up.
One of the reason for taking some pills is that for me it can become a kind of out of body experience, the process leading up to an attempt. And so it can become in one's mind depersonalised and there is perhaps less fear, as this is not really 'me' that is going this. Not so much that you conk out before before you go through with it.
As for lying flat, another time i tried it in the car and lying down in the back seats, with the rope tied to the railbars and then closing the door on it. There was no way this was going to move. Slipknot was adjusted to the point where the weight of my head remained 6 inches from the floor when tight to allow for sagging and i lied on my side, again with the knot on the left side of the neck. I messed this up by somehow scrabbling the thing off my neck having taken the pills, i am guessing such a thing is more likely to happen if you are horizontal. I might also have been able to put my legs off the seat and onto the floor which allowed me to get it off, although i don't remember any of what happened. In any case, i felt less pressure on the arteries with this method before passing out from the drugs. Clearly the head may weigh let's say 10 kilos and a semi suspended one more like 60. Apparently only 3kg in the right place can be enough but it seems to me that just the weight of a human head makes lying down to do this more prone to failure unless you can get it spot on, or wear a very heavy hat.....