Its a nested bellows design, theres plenty of info on these on the net. Try browsing;
www.ron-ger.de/ www.stde.ch/ EDO-04 RMV Keyed Semi-Closed Rebreather - Rebreather World Forums
Essentially its a gas extender, usually in 8:1 but sometimes 10:1 ratio. It'll extend whatever gas you plug into it. There are many clones, some claimed to be worse some better than the RB80 (try the links above).
There are NO electronics so all your alarms are OC intuitive (ie if you cant hear bubbles, its not venting so the loop isnt being replenished.. bail out).
The big criticisms of these units are;
1. Poor choice for ocean diving, they're designed to work best for long depth stable dives.
2. Poor WOB in any orientation other than slightly ass up.
Like any breather, its a tool...... but unlikely to suit diving for a UK ocean diver. A bit like bringing a banana to a knife fight (always a bad idea, unless the banana is to bribe king kong into squashing your opponents).