All sorts of things are not aligned with the patent exactly. There is no set screw on the collar. From what I could tell, the collar is tall enough to be approximately the same height as the resting position of the center pivot. Meaning that you’re presumably just supposed to bottom-out the tool and have the bit dangling in the air on the backside of the cut.
The instructions I thought were pretty clear. You don’t need a depth stop as this is supposed to be full-depth the whole way.
My plan to test this was to suspend some 3/8” ply between a couple saw horses and just let the circle fall to the ground when I get through (or if I was smart, do an interrupted cut to make a tab for a helping hand or two).