Originally Posted by
BAengineer
Walked out on to the wing and selected the refuel valve open?.
But seriously as you hint at - you cannot transfer fuel between tanks in flight. All you do is use center tank fuel before wing tank fuel - which is the normal operation of the system.
Actually on the B777-300ER, when either main tank is below 23,900kg and the center tank is below 15,900kg. A scavenge pump will transfer fuel to the main tanks. All the crew would have to do is turn off the center fuel pumps early. With these conditions being met, transfer would begin immediately. Normally the center fuel pumps aren't turned off until ~1,000kg remaining.
Note, the last 100 or so deliveries have slightly different trigger values.