There's a little button thing, just inside the filler. When you screw in the cap, the inner edge of the cap presses against it allowing air to circulate between the top of the tank and the filler. I've fitted a small length of rubber tube between the edge and the button thing, so it's permanently pressed. Means you can add about an extra 7 L of fuel...
Also easier to accurately measure MPG, as brimming it with the button up gives false fill readings. Brim it, then press. The fuel disappears!