Flush routine: rinse bowl a little bit (water or SkyChem), the suction starts, then a flap inside the drain which is unseen normally, closes. the flap adds diffrental pressure. which conserves the vacuum generator (ground alternitive for diffrental presure) unless on the ground (diffrental pressure is a vacuum maker, it goes at start of flush in the air) , if tank gets full vacuum won't work (fun fact: if one vacuum toilet clogs, it disables any of them near it, typically 4 to 6 or all)