Typical retrodirect gearing gives a two speed fixed gear bike. It uses two single freewheels and the chain is routed so that you get one gear ratio while pedaling forward and a second gear ratio while pedaling backward. The bike cannot be rolled backwards, though, since the two freewheels fight against each other.
This is a very interesting gearing system, but getting used to pedaling backward takes some time.
There is a Yahoo group dedicated to retro-direct gearing and bicycles, but it has been silent for weeks.