Your 2nd stage will fill with water any time it's out of your mouth. Before you take a breath from the flooded 2nd stage, you must clear it using one of two available methods.

To use the blast clear, all you have to do is exhale through the 2nd stage. This forces the water out through the regulator's exhaust valve.

The purge clear is used whenever you do not have enough air left in your lungs to use the blast clear. To use the blast clear, push your tongue against the mouthpiece and briefly press the regulator's purge button.

Remember, you must never hold your breath when breathing underwater. Therefore, whenever the 2nd stage is out of your mouth, you must maintain an open airway by exhaling a small stream of bubbles.