retro-direct 2 speed
Tarik Saleh had some photos of a Chinese work bike (well, trike actually) with the funkiest drivetrain I have ever seen. 2 speeds, high and low. You get high gear when you pedal forward, low gear when you pedal backward! A creative blend of genius and insanity.
In perusing (scouring? consuming? imbibing?the Clockwork site, I happened across a photo of a bike with this mythic drivetrain that makes it clear (as mud) how exactly it works.
2 separate freewheels, big and small, across which the chain moves in opposite directions. One freewheels while the other is endaged. Forward gets the small cog, backward the big 'un.
The image above had Hirondelle in the name, so I googled for that and found that this gearing is called retro-direct, and has a bit of a cult following. The wikipedia entry suggests that it is the high gear that is engaged with backwards pedalling. That might make more sense, and it would certainly look cool to see someone fly past pedalling backwards!