A bicycle modification
that makes the bike go forward
when you pedal forward
and the bike move backward
if you pedal backward
The scary part about this is the lack of a braking mechanism. Some folks like to add a front brake but it seems pretty silly to me. The way to stop is to ride out your momentum or to try to reverse the direction the cranks are going in (I do not recommend this).
As crackpot as this idea may seem it does have practical applications. Bike messengers tend to favor these bikes because they conserve momentum. Learning to steer instead of braking is one to avoid wearing yourself out on your first six tags.
Another nice thing about changing over to fixed gear is the immediately noticeable difference in the weight of your bike. One of my mountain bikes lost almost six pounds in the transition which doesn't seem like a lot until you've lugged that six pounds through many office building hallways and up countless flights of stairs. On days like that you feel like emptying out your water bottle to save a little weight.
Steer! Don't Brake!