Sorry, I realize I've come to this a little late, but I suspect the Gibbs is intended to use a short, 'single loop' belt set-up that hangings from the trigger guard swivel. I imagine the shooter would adjust the belt so that when prone, with his left forearm placed through the loop and holding the fore end, he is able to exert tension on the belt (and thereby increase his shooting stability) by pulling the butt stock back into his shoulder with his trigger hand.