Mike Haasl wrote:If I'm now correct, I think the post is the spine, the legs go up in the air and the arms reach down to the ground?
That's right.

I didn't want to start off the description with, "Now, imagine someone is standing on their head, their ass pointed skyward..."
So yes: hands down on the ground, feet up in the air. To extend the description a bit more: the shelf pieces are attached to where the shins would be.
This document isn't meant to be the main descriptor tool (though I think it does all right at that). It's a supplement for "classroom discussion." And I wasn't able to fit much on a single page, including the diagrams. So there are minor details left out that are clarified in-person and once someone sees an example rock jack on-site.
"We carry a new world here, in our hearts..." --Buenaventura Durruti
"Don't wish it were easier. Instead, wish you were better." --Jim Rohn