What is the working principle of the toilet eight-character valve?

Mainly the siphon principle. After the water fills the elbow that goes up first and then down, the water in the sewer pipe will turn over the upward elbow and be pumped down with the water on the other side of the elbow that is higher than it (that is, the part you can see). This kind of up-and-down bending is not easy to see outside, but hidden inside. For example, the water in a higher bucket passes through a pipe filled with water, the water outlet of the pipe is lower than the bottom of the bucket, and the water inlet of the pipe reaches the bottom of the bucket. In this way, the water in the bucket will follow the pipeline, cross the edge of the bucket mouth higher than the liquid level, and flow to a lower place outside the bucket, that is, up first and then down until all the water in the bucket is used up. This is called siphon phenomenon.