A scuba mouth piece works by reducing the air pressure in your dive tank from around 200 bar (2,900 psi) down to ambient pressure. Ambient pressure is the pressure of the water that surrounds you on a dive. The 1st stage vs. 2nd stage of your regulator is where the magic happens. The first stage reduces the pressure to around 8-11 bar (116-160 psi) or intermediate pressure. The second stage is where the intermediate pressure is reduced down to ambient pressure, where it’s safe to breathe.