The 2nd stage is the component you hold in your mouth, and delivers air on demand when you breathe. When you inhale, a flexible diaphragm is pulled in which opens the 2nd stage's valve. The valve closes when you stop inhaling. When you exhale, the air exits through one-way valves located underneath the mouthpiece.

The 2nd stage delivers air at ambient pressure, or the pressure equivalent to your surrounding pressure. This means that no matter how deep you are diving, the regulator provides the amount of air you need to breathe.

The front of the 2nd stage has a button that pushes in on the diaphragm. This allows you to clear the 2nd stage in the event it fills with water. Your instructor will teach you how to clear the regulator during your pool sessions.