Please explain those? I've never heard of them and google did not enlighten me.
Jackass meaning improvised in a fairly strange way. I stood and held on the post of the covered line with my left hand and held the AR with that hand as well. More stable than offhand. Royce was seated with the magazine rest on the bench but not stable at all. A serious shooter would not have done it in a serious application.
There term Jakass was used for such position in the "Art of the Rifle".