The yellow arrow points to a leather cord tied similar to a poacher’s knot to have an end loop.
The red arrow are ends of an elastic string which passes through a hole (one for each end) in the leather.
I need to tie the elastic string ends in such a way to secure both the leather cord, and on itself so that when the elastic is pulled, it will not stretch. Also it should be hard but not impossible to untie.
Also it should not be too bulky.
I get you on that, it doesn’t look like much. I’m guessing that’s because it’s a true follow through & looks “traceable”?
Physics or whatnot means the stress that knot puts on itself allows it to retain roughly 80% of the ropes strength, without a backup knot.
Many use a backup knot, but that’s typically just for convenience, to get the loose end out of the way.
The biggest reason, aside from the knot’s strength (…and to my knowledge), that it is used for climbing, is because it will not fail or slip past a rope delivery system, such as an ATC or a grigri.
Basically if you’re ass backwards & heading in the wrong direction, that knot may help stop your decent by not blowing past a safety measure.
I think I made an unclear comment, lol. I actually meant it in a good way. It looks like a strong knot that holds up until it’s time to take it apart, in which case it would be somewhar easier than a bowline.