The letters SV stand for Scupper Valve. How it works? When the diver clears the SV1 a small watertight tube located inside the snorkel body is designed to keep the breathing airway path free of collected water droplets. The one way valve orScupper Valve simply allows water to drain into a lower reservoir each time the diver exhales. The reslut is a crisp clearing, hydrodynamic, snorkel without being big, bulky, and uncomfortable to use