There are two types of auxillary fuel tanks, one that transfer fuel from auxilary to the main tank when the button is pressed (Have seen it in Nissan Patrols). While the others simply shift the fuel usage directly from auxillary tank. My 1FZ100 does the later type, i.e fuel remains in separate tanks, pushing the button merely changes which tank supplies fuel to engine! When you press the button, the sub-tank indication lights up on the guage and fuel level in aux tank is shown, and level will only go down if you keep using it/ driving it,
Both tanks do have a inter connection at the top end somewhere, because if parked on a significant incline, some of the fuel does go into the other tank!
