The bike grinder works kind of like this: you sit on a seat and there are handlebars at your side for stability. You pedal and the chain is linked to an arm that spins a corkscrew which pushes whatever is in the basin toward the mill stones. You can adjust how far apart the millstones are from each other to control the fineness of the end product. It doesn't quite feel like a bike because you really get stuck at the 12&6 position. Justin said that he plans on adding a flywheel which is driven by your pedaling and in turn drives the grinder/corkscrew arm.
It was pretty fun to make my own PB. It tastes pretty good too. Only one ingredient (if you don't count man-power!). They could make flour there or I've heard about a possible blender attachment for making smoothies. I think it's got potential!