The flap valve just lets the water through, all in one go, no siphon, you move the lever, the flap lifts. if it’s just a little pee to deal with, you can release the lever early, that’s the water saving feature. How much water is in there is a function of the height you set on your filler valve.
These things work, I have owned a bunch of them for many years now (I’m a landlord). Bonus feature is that if the flap ever needs cleaning or changing, you don’t need to separate the cistern from the pan to do it, and replacement parts are available though my oldest one is at least twenty years old and going strong.