instead off having a freewheel on the back on a normal bike, the freewheel is on the front on the cranks, so when the fixed (fixed= A cog that is screw onto the rear hub that doesnt move either way) reear wheel spins it drives the freewheel at the front and the chain moves when you dont pedal, Pedal backwards the chain doesnt move Freehub is the freewheel built into the back hub, and instead off a freewheel on the cranks you have a fixed sprocket so when you stop pedaling the chain does not go round, like front freewheel This is more similar to a normal mountain bike, pedal backwards the chain goes backwards.