This knot is quite a strong knot. I have used it to tie the top spar of a balanced lug to the halyard. It held and is still holding just fine. Its quite hard to take apart. You need a knitting needle or some kind of instrument to pry apart the strands. Even then it's easier to cut it.
Tying a Constrictor Knot
To hold this knot needs something behind it. Round and oval shapes are best.
I've made it here with big rope on a quite small dowel so its not in proportion but with smaller line it is one tough little knot. It took me for ever to learn this one, just didn't want to go in. There are many ways of tying the constrictor knot and a few variations. I first met it when I was working on a farm. It was used to tie up bags of grain, it was called a miller's knot then. It's well worth learning.
email me if you find mistakes, I'll fix them and we'll all benefit: Christine