What is the principle in the Baermann funnel method of extracting nematodes from the soil?

The Baermann funnel method is based on the principle of using gravity and water to extract nematodes from soil samples. In this technique, a soil sample is placed in a funnel above a container of water. As water seeps through the soil, nematodes, which are motile and tend to move downward in search of moisture, are encouraged to migrate into the water below. After a period of time, the nematodes can be collected from the bottom of the funnel for further analysis.