I thought that Les Stroud was about to make a taut line hitch. But at the end he did a couple of things a little different. What is this knot? Does it hold better than a taut line hitch?
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This knot appears to be one referred to as the tarbuck knot, named for its inventor, see for example: https://en.wikipedia.org/wiki/Tarbuck_knot or http://www.craigmarine.info/accessories/fishing_equipment/Tarbuck-climberKnot.htm
It is intended to be able to absorb sudden loads, which is useful for climbing, especially in stranded, synthetic type ropes.
(Note: adding as community wiki, as the correct answer was found by another user, but that answer was deleted for reasons of moderation.)
It seems related to the knot that I use to tighten a tarp ridgeline that is often referred to as a 'prusik'. I can't remember how I first came across the method but it's very useful for maintaining tension.