External catheters let urine flow by gravity into a collection bag. Urine still tends to stay in the catheter and tubing until you remove them. The Norela EvacValve sits between the catheter and the drainage tube.
When urine flows into the tube, its weight opens a small valve. Air enters, the line drains, and you can disconnect with less leftover fluid and less spillage.
This is the core of every Norela system.