Why do toilets bowls most notably in America fill up during a flush before the water has appeared, and then almost empty before rising to the normal level?

Question

For context toilets in the UK just flush, the water level slightly increases then returns to the normal level. Also watched a Cody’s lab video about flushing mercury and seen the mercury level rise before the mercury appeared.

In: Engineering

Answer 1

The water enters the bowl via two paths. The main flush comes from a bottom jet which shoves the waste towards the drain. The bowl also has holes around the rim which help clean the bowl. A proper flush requires the water to create a siphon in the U shaped channel. The U shape is a trap which keeps sewer gas from entering the home. That sucks the water level down before the bowl fills back up. The water level in the bowl ends up at the height of the U on the exit side.