It's permanently attached to rear fork and slides around to immobilize the rear wheel. Of course, you wouldn't want to use it alone - you'd want a U lock around the front wheel/frame/object as well.
This combo is what I was given for a city bike in europe, and it's painless to use - you just turn the key and slide it through the rear spokes, and the way it's attached it's hard to cut without removing it from the frame first, unless you don't mind damaging the bike or wheel.
http://www.amazon.com/ABUS-Amparo-4850-Frame-Lock/dp/B001BAD...
It's permanently attached to rear fork and slides around to immobilize the rear wheel. Of course, you wouldn't want to use it alone - you'd want a U lock around the front wheel/frame/object as well.
This combo is what I was given for a city bike in europe, and it's painless to use - you just turn the key and slide it through the rear spokes, and the way it's attached it's hard to cut without removing it from the frame first, unless you don't mind damaging the bike or wheel.