The idea is that when you fill up and the fuel gets to the level of the totter, the fuel rises very quickly in the totter itself which causes the pump to automatically stop. Remember when you fill up the pump stops automatically when the level reaches the nozzle.
If you continue filling up very slowly (but takes a lot more time) you can still fill the whole tank, you just have to let the fuel seep down (and let the air go out the breather).