tire chains for additional info. There are probably other brands as well but the operating concept is likely to be similar. Briefly, the arm rotates so that the rubber disc presses against the tire sidewall. As the tire spins the chains are slung out away from the disc; with enough speed the chains swing out to where the tire can run over them. Because the whole works is driven off the tire sidewall the movement of the tire tread and chains are synchronized so it doesn't tear itself apart.