Simply the SP creates a unique picture to the eye over the rib, it has been designed to do this in your peripheral vision so your focus can stay on the clay. There will be a small profile unique to one set of barrels, this will identify the correct barrels much the same as if you had one set of barrels – like a shooter with no dominance issue. (they will have one set of barrels from their very strong eye and a ghost or no image from the eye off the gun).