Bob Thrun invented this ascender while researching semi-mechanical ascenders for his book Prusiking. In effect, he saw what others had done and realized that creating semi-mechanical ascender designs was almost too easy. He mentioned this one, which is arguably the simplest way to make a semi-mechanical version of the Prusik knot. I made mine from some hardware store aluminum. Strength is not an issue - if it bends, it bends; if it breaks, I've still got a Prusik knot on rope.