This is an image for Photos.

One of the things the grad student from Rice showed me was a tensioning system that uses only carabiners. What makes it special is not that it gives you a mechanical advantage of four, but that it will hold itself in position when you let go after every tug (in this picture I’ve tied and wrapped the excess around the interesting part of the system, which makes it hard to see, so I apologize). Basically, you run the webbing through the ‘biners so that the last bight before where you hold it passes under a prior one. When you pull, you’re putting more force on the pass going underneath, which pulls the ‘biner away from both and lets the last pass run freely. When you let go, however, the underneath bight is sandwiched between the top bight and the ‘biner and is held tight. (If you want to understand why they don’t just both slip, remember that with any block and tackle the ropes passing through the pulleys will be moving at different speeds. Because the underneath pass in this example is the last one before your hands, it moves much faster than the earlier bight passing over it, which is how the earlier, top one can catch it in place. The top one catches it, which seizes movement in the entire system and keeps them both still.)

The grad student who showed me this didn’t explain how the system worked (he was having trouble just remembering how it went together) but it’s pretty easy to see by observing the thing in action.

Hooray for friction!