Sonic ventilation

The Studor valve has a light spring that keeps the diaphragm seated to keep drain smells contained. When the drain is used, the diaphragm will open if needed, to prevent the vacuum developing and slowing down whatever is going down the drain. Once the flow is gone, it reseats. They are not perfect. They can get trash, mold, or cock to one side sometimes. Most are just screwed in by hand, for an easy change out.

The link provided in my last post goes into more detail.