Ok I give up, I don't have any more info on this, but somehow it detects pressure (if you pull the nozzle out the filler neck you can put more fuel in). You'll need a reply from someone who works on the pump design, I work on the car side. Sorry.
I like the elf idea (do you have to go to an Elf petrol station for it to work?), but there are enough gremlins in cars these days without adding elfs.
The fuel tank has a dip tube which keeps a void at the top of the tank (~10% vapour space). This is important since the fuel is pumped in cold...