So why not shut their air off and purge the regulator and then allow them to do the CESA? They would learn to keep the regulator in their mouth, so they don't "inhale water and start to drown" and you can make sure they are not cheating?
Two separate reasons, that kind of figure back to the same original reason:
One: Turning off the valve does not teach a diver how air runs out in the real world. In the real world someone "Out of Air" has the volume of the tank, not just the volume in the hose to breathe from, so the extra breath they get is real and significant. It is in fact why people can dive without gauges, and did so in the past, and not have to do panicked ascents when breathing the very last of a tank. Because once ambient drops below the remaining tank pressure there is more air left.
(Of course this does not help with dealing with catastrophic gear failure like a yoke retaining bolt, or a second stage lever failure.)
Two: What BoulderJohn said. PADI has their head up their heinie when it comes to the CESA.
a. PADI asks that the CESA be mastered in confined, swimming horizontal, when
all of the teaching points are lost. There is no air expansion so the instructor has to swim alongside the diver adding air to their BCD with the inflator so the divers do not go negative to counteract the loss of buoyancy from the exhalation.
b. PADI also foolishly asks us to guarantee that the teaching point of oding it vertically is lost because they do not allow an inhalation near the surface, which would bring home the point that "out of air" usually means cannot get the remaining pressure out of the tank due to ambient pressure being equal to residual tank pressure.
Overarching idea here:
This is not the instructor forum, but its worth pointing out that experienced instructors have often driven standards changes. PADI HQ is no longer in the business of teaching OW (unlike the days of the PADI Dive College, or whatever it was called). That was the whole driving force behind the updated OW course in the first place. BoulderJohn et al put together a well-researched convincing case that said the 'standard' PADI kneeling teaching position was not just ineffective, but actually counter-productive to teaching bouyancy.
I have influenced PADI Instructor Examiners (the persons) far more than Instructor Examiners have influenced me in the last twenty years. Watching the change in how they conduct themselves with regards trim, to give an example, has been an eye opener. Of course the CDs are still stuck in the past. Watching a course director try and figure out how to teach the new OW system, or even do the skills is pretty hilarious, it if weren't so sad.
Also in a PADI update, the PADI Area Reps pointing out that making OW students kneel in the pool is boring to the students and is a waste of the student's time comes from certain PADI members pointing out that out to them, not from them discovering it themselves. PADI simply no longer teaches OW, and we working instructors do. They were completely unaware that they did not show anything but kneeling in CW in the old materials. We working instructors were well aware of it, having to fight to reprogram the bad first impressions OW divers got.
---------- Post added June 29th, 2014 at 10:18 PM ----------
Nice except if they don't get to the surface you now have a student with empty lungs AND no immediate way of breathing. It's a ridiculous skill but i cant think of a single valid reason to do it without a reg in, either simulated OR real.
Hopefully at least some of the divers doing CESAs are getting to the surface with mostly full lungs from the expansion on the way up. Guys with bigger lungs can usually be coached into being able to manage it pretty easily.