I've wrestled with this type of flush in the past but now I know how it works. The display board shows that the central pipe is double-headed inside the flush mechanism. One side is attached to a sealed section with a plunger in. Pulling the chain pulls the plunger up and drops water into the pipe. The rush of water past the open end of the other side of the pipe reduces the air pressure. I think this might be Bernoulli's Principle but it's been a long time! Thus higher pressure air above is able to push the rest of the water up and over. There's now nothing to stop the plunger sinking back. The ball cock has dropped in the meantime, opening the tap and letting water flow in again.
