I need help with routing the fuel line, it'll touch those red cogs on the engine?

Can somebody explain how the gearbox work, I have built it according to instructions, but I don't really understand what makes it shift and how it really works?
I guess the two red cogs have constant speed from the engine when clutch is engaged, but how does the clutch then select which of the black cogs (43&47) on the drive train that will drive the shafts?