Used to remove the cassette lockring when bike shop resources are not available, this tool secures the lockring from turning by locking it to the dropout so that when the cranks are turned with the bike in low gear the pedaling force will actually break the lockring free. It is designed primarily for emergency use. The tool can be used in reverse to be able to install the lockring as well. Shimano compatible - not for Campagnolo.
I used it on my Surly Long Haul Trucker. It worked great and I will be glad to have it on tour with me in case a spoke breaks on my rear wheel drive side. I'll be able to fit a new spoke and keep riding.