I have experimented with adjustable hitches, and 14 falls and an extremely sore a*se later, I came to the conclusion that there is not an adjustable hitch with the strength to hold up a persons body weight for any length of time, and with the ,movement that occurs during sleep.
If you examine the knots/hitches used to tie up hammocks, you'll notice that the tension is applied perpendicular to the line that the pull is being exerted on. (If you put your forearm in front of you parallel to the floor, and push against it upwards with the cup of your hand, that is how the knot works for the hammock).
This means that the tension is applied AGAINST the line of the rope. In an adjustable hitch, the tension is applied ALONG the rope, but, through the tie of the knot, it is placed at an angle to the rope. This is why it is adjustable. When you adjust the knot, you are simply putting the pull directly in line with the rope around which it is tied, allowing you to move the knot up (or down) the rope.
All this means that the knot has a tension limit before it will slip, unlike the fixed knots that have only the breaking strain if the rope to restrict them. Basically, when you put too much tension on the knot, it changes the angle sufficiently for the knot to move up or down the rope around which it is tied, resulting in a bruised backside/pride.
I hope all that makes sense (I'm not sure if even I understand it
)
Jake.