How is the anti-reverse on/off control used?

The anti-reverse switch is used to activate or de-activate a clutch inside the reel that controls the spin direction and capabilities of the rotor. When activated, the clutch prevents the rotor from turning backwards during a hookset. Some anglers prefer to de-activate the clutch to allow "back reeling" for controlling a fish, or let line play out when a fish makes a hard run.