As far as I’ve been able to figure out, the reserve switch enables the fuel pump to run when the fuel gauge points to “E.” Switched to the ON position the fuel pump will switch off when the fuel gauge points to “E,” but the manual says that 1.3 gal. remains in the tank. Switching to the RES position will allow the fuel pump to run again and pump all the remaining fuel out of the tank, a little like a manual petcock does on a bike without a fuel gauge.
Scott