oh, you ride fixed gear? yeah, i've done that....

http://myspace.voo.be/pcoupard/retrodirect_drive/

What's a retro-direct bike?
When the rider pedalled forward, the chainring would pull on the small cog and engage its freewheel, while the large cog would freewheel in reverse. Conversely when the rider pedalled backward, the chainring would drive the large cog forward and let the small cog freewheel.

With video!

I know it's going to make a horrible noise, but for some reason this seems like it would be absolutely awesome on a mountainbike.