Plug that helps hold alignment when removing and installing cassettes on bikes. Simply slide cassette onto the plug. When ready to install slide the cassette off the plug back onto the bike. I have tested on my 11 speed shimano xt cassette as well as my road sram 11 speed. I think will work for any 10, 11, 12 speed shimano style hub. Can be oriented in any direction but works best to line up one of the teeth on the plug with the narrow slot on the cassette.