Exactly as the blurb says. Fill to the line with degreaser, snap it over your chain, and rotate the crank furiously for a couple of minutes.
It doesn't return your chain to a brand new appearance - but it does do what it says: Scrubs and cleans the chain.
If you're a dedicated bike cleaner type of person, you'll find this a great timesaver, and if you're lazy - even more so. If you don't mind the rags and scrubbing brushes type of approach, then this isn't for you, and to behonest, probably doesn't do as good a job as the manual labour intensive methods.
Two things to note: Once you've de-greased your chain, you need to remove the de-greaser, dry and re-apply oil. That's not an automatic process.
One reviewer stated that the bristles were pulled out - I can't say I've had that problem myself.