Having consulted with a world-leading expert on water survival at a SAR symposium a few years ago - the mouthpiece needs to be in place before you enter the water since the human 'gasp-reflex' will likely render you unable to purge the mouthpiece if you wait until after water entry and the colder the water, the worse it gets.
Purging upside down is an art and needs to be practised in a pool not a classroom.