Isnt there an old saying ... somethign like... sufficiently developed technology is not discernable from magic (and the other way round) or something?
Either way. The idea is 200 years old and really really simple. Once you get it. I didnt believe it til I tested it, either.
I didnt find pictures, Ill call him sometime the next days and will ask him. I know he has them (same with the one-arm-guy-fully), I just dont know where to find them online.
Simplest explanation with words would probably be:
the chain is routed in a way so it pulls from the front on ONE sprocket and makes the OTHER one freewheel (at double speed, obviously) if you pedal normally. If you pedal backwards, its just the jobs of the sprockets are reversed. ie, the one that was driving before is now freewheeling. If you dont pedal , both spin freely.
Damnit, without describing the way the chain takes, its useless to put it in words
edit: just found an US patent, that does the same trick about a dozen times as complicated. I'll keep looking
edit 2: there it is:
http://www.patentstorm.us/patents/6695332/description.html too bad the bigger version of that picture is only vierwable by registered users. But thats basically the same thing as the one my friend came up with, if I see that correctly. The stuff described in that text is a bit funny. The problems they mention can probably not solved by this kind of drivetrain. It just helps keeping the stress on the body down, which is important for that particular customer of my friend (the guy had an accident and his knees start to hurt quickly). I would NOT want to use it to ride my bike uphill "more safely". Seriously.
edit3: DAMNIT. Still not the same design. Still uselessly complicated with that "auxiliary chain". But were getting there.