The standard name is: return water bend, Basin Waste Pop Up because the curved part can store water and prevent the odor from coming back. All downpipes have this structure, such as toilets, under the kitchen sink, etc.

If it is straight, it will often block,
Reason for bending:
1. The establishment of the anti-water bend requires the maintenance of the use of the pipeline.
2. The anti-water bend of each house is so that the water flow of each house will not flow straight down, and long-term changes in water pressure will easily cause pipe damage.
3. The general sewer pipes are connected to the sewer (directly or indirectly). The sewer smell is very heavy. If you use a pipe without bends, the odor will come directly up, and now the bend we see is just right. The function of water storage thus keeps the odor in the tube so that it cannot escape.