I learned this knot from a Canyoneering class taught by the Mazamas in Portland. It’s a very cool class and lots of Crafty Rope Tricks (CRTs), highly recommended!
Let's start with a little honest disclaimer right off the top: This bit of rope wizardry is probably not something you will use in very many climbing situations. However, if you’re an aficionado of Crafty Rope Tricks (CRTs), well, this is one of the craftiest you’ll see! It's halfway between a knot and a magic trick. (The day I learned it, I tied it a dozen times and was still scratching my head as to how it works.)
The voodoo hitch is a way to add a moderate amount of easily adjustable tension to a fixed rope. (It's somewhat related to the trucker’s hitch, an extremely useful knot that just about every outdoors person should know. Here's a nice short video that shows how to tie a trucker’s hitch.)
The voodoo hitch known by a few different names. Some call it the rather boring “Transport hitch”, others the “PM hitch”, which stands for “Pure Magic”. But my personal favorite, and the way I learned it, is the “voodoo hitch”. You'll learn why it's called a voodoo hitch after you tied a few times and see how it miraculously holds its shape, when everything you think you know about knots tells you that it shouldn’t.
Just get a climbing rope and a couple of carabiners and try out this rope sorcery. It's cool, magic, and fun.
Here’s what's nifty about the voodoo hitch (and how it differs from the trucker’s hitch:)
You can fine-tune the tension on the rope without untying and re-tying the knot.
There's no lock off half hitch required, the voodoo just magically holds tension by itself.
A trucker’s hitch creates a 3 :1 mechanical advantage; the voodoo hitch (I think) creates no mechanical advantage. So, if you really need to crank on a rope to make it seriously tight, the trucker’s hitch is probably a better choice.
A few notes:
I was taught this knot in a canyoneering class using all clove hitches. Clove hitches work, but there are several other knots work fine too, such as a butterfly, figure 8 on a bight, or an inline figure 8. I’m liking the butterfly knot, because it's easy to untie after it's been loaded.
You can use any kind of carabiners for this: lockers, non-lockers, large belay lockers or tiny wiregates.
It works with some kind of a opposing tension or friction or voodoo or magic or something. And you can decrease the friction using pulleys and it still works great. I put a pulley on two out of the three redirect points, and that worked, but a pulley on every redirect point, and it doesn't work. Like I said, voodoo!
Here’s a good article on the Voodoo hitch, along with this nice graphic:
As with pretty much anything related to learning knots, this is a better show than a tell. Check out this short (2:20) video that shows you exactly how to tie it.