Between the forward and reverse gears of an outboard gearbox sits a clutch that is attached to the shift shaft, with the prop shaft being attached to the clutch. When the shift shaft is in neutral, the clutch isn't engaged with either the forward or reverse gears, so it remains motionless, as does the prop shaft. When the shift shaft is put into its forward position, it engages the clutch with the forward gear, which causes the clutch (and the prop shaft) to rotate in one direction. When the shift shaft is put into its reverse position, it engages the clutch with the reverse gear, which causes the clutch (and the prop shaft) to rotate in the opposite direction.