What plumbing feature is essential for ensuring air flow and preventing negative pressure?

The essential plumbing feature for ensuring airflow and preventing negative pressure is venting. Venting systems are designed to allow air to enter the plumbing system, which is crucial for maintaining proper pressure and preventing the formation of a vacuum. When waste is removed through drainage, it creates a negative pressure in the pipes if there is no venting. This negative pressure can lead to slow drainage, siphoning of traps, or even the backup of wastewater. Properly functioning venting ensures that air can replace the volume of wastewater being removed, allowing for smooth drainage and preventing any backup in the system.