There are martingale rein stops for running martingales that go on your reins. The martingale stop that you have pictured goes across the intersection of the 2 straps of your martingale. How do you get them on? I find it's easier if you soak them in hot water for a bit to soften the rubber and then use 2 hoof picks to stretch them out over the loops. Also helps to swear like a sailor and glare at the horse that needs the martingale.