Your free leg crosses in front of the standing leg, between it and the wall. The foot presses the wall on the far side. As you stand, your hips and chest corkscrew until every limb is straight [140].
Grade Gripped magazine calls it rarely efficient but occasionally ideal [142]. It starts to pay at about V4.
How to do it
Stand on a single low foothold, below knee height, under your holding hand [140].