There are two types of exit hole, small on the first generation and large on the latest. The flush works in two stages, first a small quantity of water is released then a powerful vacuum is activated for around three seconds. Passengers waiting for their turn have to wait longer for the present user to flush few times on attempts, often unsuccessful, to clear their mess. To leave it clean it is best, before use, to apply a coat of toilet paper on the bottom and side of the bowl. A known incident, or popular joke of questionable taste, about an obese person been vacuum sucked in the toilet bowl is mostly improbable as the flush push button, behind the user back, is not easily accessible.
Vacuum toilets are fun too.
Some flight attendants, when passengers are not yet on board, compete on how fast they can flush away a pre-unrolled length of toilet paper. Parallel races also take place. Vacuum toilets, with their small connecting tubes, are badly clogged. Some peoples seem to enjoy the toilet hard throne better than their allocated soft seat while frustrated passengers, in urgent need, wait for their turn. Knocking helps, however the most efficient way is to unlock the door.