Negative pressure in a DWV is created when a drain or fixture has fluids flowing through them, this is why a vent is required. Without a vent the water would flow very slowly or not at all. Example: stick a straw in a glass of water and before you remove the straw, put you thumb over the top of the straw. When you remove the straw, the water stays in the straw because there is no vent. When you remove your thumb, the water flows out of the straw because you have a vent. The Sure-Vent allows each plumbing fixture and drain to receive the needed air to drain properly. Positive pressure is present when the city sewers are relieving pressure that is designed to push back up through the building sewer and eventually out of the roof vent on your home or building. While this positive pressure is being released and your plumbing fixtures and or drains are not being used, the Sure-Vent will remain closed. This is important because if the Sure-Vent did not protect you from positive pressure, your home would fill up with foul odors.