Regarding bcd rebreathing: I've dove my rebreather with no scrubber on 21% air (no o2 added, strictly semi closed circuit) out of curiosity of this tip. It is a horrible experience of co2 build up and one of the toughest mental battles I've had on a mundane dive
I agree rebreathing stale air does not seem the path to effective problem solving.
But from the mental and physiology standpoint, why did this let you survive, reasonably in control, with less air than OC breath holding? The CO2 your lungs see seems almost the same. The differences I see, from limited knowledge
from basic reading, seem to be:
1) You still get to instinctively exhale and breath in more. It just doesn't do you any good until you add fresh air to the bag. From my earlier reading of static apnea, stillness or motion of the diaphragm plays a role in the extreme stages. On OC, this would be using more tank air on each such breath, not just the ones when you allow yourself fresh air.
2) When compared to OC breath holding, when you vent and get fresh air, your first few breath cycles on that air can reduce your body CO2 into the same volume of fresh air, for a short while, while on OC that would take a fresh lung full of air each time.
How does this differ for BC rebreathing:
1) Breathing stale air with high CO2 from a rebreather loop gives you a solid mouth piece attached to you, while your BC manual inflator does not. So its possible to keep doing it longer into panic stages of blood CO2.