This ingenious device allows you to get back home should you break your hanger out on the road or trail. Simply remove your broken hanger, derailleur and QR skewer nut. Put the QR skewer through the Emergency Hanger, remove the spring and re-attach the nut. Secure the QR nut and skewer down tightly. Re-attach the derailleur and away you go!
In most cases you should have the ability to shift throughout half (or more!) of your rear cassette range. This hanger is designed as a quick, Emergency fix to get you back home and is not designed for long term use.