After flushing the toilet, water enters the bowl by way of the rim holes and jet.
Water rises in the bowl and flows over the dam, but no siphon or flushing action has yet started.
The curtain of water also prevents air from entering the passageway through the outlet. As incoming water continues to accelerate, more of the air in the down leg of the passageway is displaced. When the passageway is filled, a good flush or siphon action is created. As soon as the level of the water in the bowl drops to the level where air is again introduced into the passageway, the siphon is broken. When deep seal of water is not restored with refill water, sewer gas will enter.