Overgrowth: Backwards Ledge Grab
The map used in this video is Red Canyon by elegaro. This is one of the more common/reliable bugs in the Overgrowth Alpha (I am playing a199 here, but this has been around for a while). It works because of the ragdoll physics implemented in this game. When you grab a ledge it puts specific body parts in specific places, facing specific directions. The rest of the body should follow fairly easily and realistically. When you are inside of the ledge (a slightly common glitch), it still puts those body parts in the right places, but the right places are behind you. The rest of the body cannot follow suite, so the ragdoll does its best to fit.