Earlier you learned that it's not likely that vour regulator will fail so that it would cut off your air, but that a failure would most likely cause an air free flow. You can breathe from a free flowing regulator if you don't seal your lips on the mouthpiece. During this confined water dive, your instructor will have you
practice breathing this way

Since your regulator probably won't cooperate by spontaneously malfunctioning right when you need to practice this, you'll simulate the free flow by (you guessed it) holding in the purge button.

Remember to breathe withont sealing your mouth on the regulator, "sipping" the air you need while allowing excess air to escape. A free flowing regulator can really rush - don't be surprised if it jostles and floods your mask a bit. You'll breathe from your simulated free flow for at least 30 seconds, and your instructor may have you practice turning off your air after surfacing like you would with a real free flow. If you can't reach your tank valve unless you remove the scuba unit, do so for practice. Although your buddy might do this for you, doing it yourself develops self-reliance. Check your SPG when you're done; you'll be amazed how much air a free flow eats up in only 30 seconds - which is why you head straight for the surface if it happens.

Controlled Emergency Swimming Ascent
As you learned, the controlled emergency swimming ascent (also called CESA - pronounced "see-sa") is one option if you lose your air supply at 6 to 9 metres/20 to 30 feet or less, and your buddy is too far away to provide an alternate air source (Buddy system, buddy system! You shouldn't be that far from your buddy!).

Emergency swimming ascents are interesting because you start with air in your lungs, exhale all the way to the surface and still have air in your lungs when you get there. This happens because air expands in your lungs as you ascend; the potential hazard is a lung over expansion injury, which you avoid by not holding your breath.