Hi !
Just take this case.
You have full fuel, Wings + Aux + Tail, all full.
You have to wait for more then 2 hours with the APU running and sucking the right Wing below your balance Limit of 400lbs for Take off.
There is no choice given from the System to get back into balance.
The only way to do is, pull the CB of the right Fuel Pump, switch on the pumps and start the right Engine.
Because the Fuel Pumps deliver more then the Engine will need, the Return Flow will logically go into the right Tank with the right Engine running.
This is the only way to get Fuel back in the Wings on ground.
Tested it, Did it and amazed a lot of people with it.
Take care