AWb,
The purpose of the open channel between the supply tanks and the main tank is to allow 'spill back' of excess fuel that is being pumped from the main tank to the supply tanks.
Just one of the two transfer pumps operating in the main tank will transfer fuel to the supply tanks at a rate greater than both engines at full power can consume. A simple way of keeping the supply tanks full until the contents of the main tank are exhausted.
Of course it works both ways as you suggest and a more extreme nose up attitude can gravity feed fuel from the main into the supply tanks at certain levels.
ie....you lose both transfer pumps but fuel remains in the main tank, allow the engines to use some of the now reducing amount of fuel from the supply tanks then pull nose up to get some of the main tank fuel into the space now in the supply tanks (taught during type conversion).