As said it goes on the wheel after the largest cog and over the spokes. If the derailer is not addjusted properly when you want to get into the largest cog in the rear with the chain and it goes beyond (too far) the cog the chain will chew up the spokes as the wheel spins. This happened to me a few years ago and destroyed a wheel. However if the derailer is adjusted properly it should not go past the largest cog and this dork disk is not necessary.