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The knot I was talking about can best be seen here
http://www.iwillknot.com/slip_knot/ I consider this such a powerful and valuable knot because it is so fast to tie (a kid can tie it in less than a second) and can be used in a lot of applications. There is a catch, however. If I was using this to send a polesaw, I would pull the loop up from the top. If I was using the same knot as a stopper to pull on a tagline, the loop would have to be formed from pulling the bight through BELOW the loop.
The same applies here. If I'm standing atop a spar, the I'd make the loop, then insert a bight from the running end of the rope (the side heading around the spar) rather than from the side immediately heading to the ground. The way the loop would tighten down on the 'biner.
All that said, I've have to try it out, first. I don't know just sitting here if the thing would slip or not.
love
nick
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Nick I have always called this knot a Marlin Spike.
I use it for sending chainsaws etc up trees. And for sending the workin end of the rigging line back up(acts as stopper knot in the block, only when it is tied the right way)
I certainly would not be promoting this as a suitable midline knot for cinching your climbing line, precisly for the reason you stated. If you tie this knot the wrong way round it does not work properly.
A clove hitch can only be tied one way so is far better than a slipknot(marlinspike). Or you could use a figure8, butterfly, bowline on bight etc.