The Hollis 221 Second Stage Regulator is a Pneumatically Balanced Demand Valve Design. The Hollis 221 Second Stages also has an Adjustable Venturi System (A.V.S). This simple Dive/Pre-Dive Switch allows the diver to Adjust Inhalation Effort so the Second Stage won't Free-Flow during shore entries or long surface swims. The Control Knob has an Easy Grip for Precise Adjustments even while Wearing Gloves. The Hollis 221 Second Stage Housing is manufactured from Durable ABS/Polycarbonate. Regulator also comes equipped with a Patented Orthodontic Silicone Rubber Mouthpiece for Comfort and Reduced Jaw Fatigue. Second Stage Regulator comes with a CD Owner's Manual and is covered by a 24 month limited warranty.