Hi. Imagine there is two holes where the fuel can go to the
engine. The “on” hole would be higher at the reserve litre’s mark
while the “reserve” hole would be at the base. So if your switch is
at “on” and the petrol level has go past the “on” hole, no petrol
can be fed to the engine. You would have to switch to “reserve” to
use the remaining fuel which is your reserves….. thats what my
friend told me.