The lever points to the tank that the status indicator bubble, red/green, is referencing. Your picture shows green, or that propane is present in the referenced tank. Transfer from an empty tank to another is automatic given that both tank valves are open. Before you disconnect the empty tank flip the lever towards the supposedly full tank and you will now be monitoring the status of that tank. When you install the newly filled tank, simply open the valve...DO NOT flip the lever unless you want to use the propane in the just-filled tank.