A sensing device, located about one inch from the end of the nozzle, does nothing while fuel is flowing into the gas tank, but is tripped as soon as fuel backs up into the nozzle. The sensing device tells the nozzle to shut off.
Because of the location of the sensing device and the relatively deep position of the nozzle, a gas tank is never totally filled unless the customer or attendant “tops off” the tank. Topping off tanks is now illegal in most states and is a dangerous practice anywhere.
From: When do Fish Sleep? by David Feldman