This device from Swenor attaches to the same holes as the fender without losing a fender. It is a roller ski brake for added safety during a workout. Make sure to adjust to height and preferred positioning of the skier prior to heading out. When rollerskiing, lunge the brake ski forward which will push the brake down on the wheel, slowing you down without running onto grass, gravel, or a curb. Only comes as one unit, no need for a pair.