How does an auxiliary long range fuel tank work?

[DeLorean]

I bought a second hand Gen 2, and only found out it had a long range fuel tank fitted when it went in for the roadworthy.

There's a switch which turns on what sounds like a pump, and then the fuel needle on my gauge seems to go up.

Can anyone explain very briefly how this works exactly?
How does it get topped up? How do I know when it's empty? Should the fuel be pumped while driving or at standstill?

[royl]

Burnco recently fitted a 65 litre tank in the 3rd row seatwell and this is how it works:

I have it filled through the standard fuel cap. The main tank is filled first and I estimate how many litres it will take, let's say 40 litres. I then tell the attendant to expect the flow to slow down after 40 litres and to carry on filling it up carefully, otherwise they think that the "click" indicates that it is full.

I can switch the pump on when the car is stationary or moving (the pump is connected to the main battery so I suppose it is best to have the engine running whilst doing it). If I forget to switch the pump off when the main tank is full it is not a problem as the extra fuel just runs back into the 2nd tank.
There is a slight difference in the pitch of the pump when pumping fuel vs air so I use this to hear when the 2nd tank is empty.

As I do not have a gauge on the 2nd tank, I monitor the amount of fuel I pump across by using the amount the needle moves on the main guage and estimating the number of litres.

After using jerry cans for the last 12 years I find it just brilliant and works so well.

Hope this helps.

[DeLorean]

This is most helpful @royl. Thank you!