As other have pointed out... we breathe air due to the "tension" (difference) between the ambient pressure and the tank pressure. You have a TOUGH time breathing on a snorkle that is longer than 18 inches because the air in your lungs is at 1 atm while the ambient pressure surrounding your chest at 18 inches is 1.045 atms which is less than a single PSI.
If the pressure OUTSIDE of the tank is 4atm, then you can not breathe your tank LOWER than 4 atms. However, bring it up to 33 fsw (2 atm) and you now have a 2 atm tension (4 atms in the tank and 2 atms outside). Breathing is now possible until you breathe down the tank to the ambient pressure again.
This is why we teach students to KEEP their regs in their mouths! You don't want to waste ANY air trying to clear the regulator so KEEP IT IN!
For those of you who have taken up the sport within the past 20 years, remember that for MANY of us during the 70s, running out of air was the way we ended our dives. We did not OWN an SPG or a depth guage for that matter. When we felt the "pucker" we would reach back and trip our J-Valve for a couple of hundred PSI to get to the surface. Often, we would find that the J-Valve had already been tripped and we just had to KICK HARD to the surface. Even then, we could always count on at least one breath on the way up.