UNLESS it is done horizontal shallow while exhaling which is a classic recipe for a blackout.
(Single data point: I can black myself out in a pool swimming underwater fairly easily. Not breathless after even a 4 minute CESA.)
Interesting. I’m skeptical that this is an issue as all students I certified under the PADI system was exerting themselves. Of course I was always right there, I taught the CESA the same way I was taught by my CD and we were tested the same way in my IE,
If students were blacking out ever, I think that would raise some red flags at HQ,
Did you black out when you took OW?
I didn’t because we were never taught it in the pool (oh the standards violations were something!)