Flushing the toilet on an airplane opens a valve between the pressurised cabin and a tank that remains at atmospheric pressure. That process creates exactly this sort of pressure differential. In other words, “when you flush the toilet, you’re basically opening a valve to the outside, and the pressure differential sucks away whatever’s in the bowl,”

The beauty of the system is that it doesn’t require vacuum pumps or other complications to create the pressure differential. It simply makes use of the existing difference in pressure between the aircraft’s interior and exterior at altitude. Of course, this requires that there is a pressure difference. On the ground, the system uses a vacuum pump. The pump remains in use until the plane reaches an altitude where the atmospheric pressure allows the system to work without it, at which point it is switched off.