The water doesn't rush back down because there's no way for air to be allowed into the system. There's a similar and much easier setup called a water bridge which uses clear pvc type piping shaped like U. They're usually a lot smaller and the way it's done is they turn it upside down so the U is facing up then they fill that up with water and cap off the ends. They turn it so the U is upside down and it is mounted, once the ends are under the water they can unscrew the caps.
It's the same principle as placing a glass under water and turning it upside down and then pulling the glass out of the water just enough so that the rim doesn't come above the surface. Since no air can get inside the glass the water has nowhere to go.
1. Can't perform a large WC.
2. Water in the tubes are probably stagnant. You won't be able to use a powerhead in one end to push current through the tube, becuase you will be pushing more water through. The tank on the other end would overfill.
3. Cleaning the tubes. Sure you can use a magnetic scrubber, like a Mag Float, but you probably have to be careful NOT to shift the tubes around too much.
4. Fish may be able to shift the tubes when they are in there.