Hi all!
firefly: What happens when you hyperventilate is that your CO2 blood concentration
decreases so much that your central nervous system response is to narrow all your
blood vessels... The result is that feeling of 'light' headness, irreality.. etc. If you do too much too fast, you can blackout on dryland due to vasoconstriction. This is called hypocapnia blackout and if it happens while underwater you risk your life.. not much fun stuff. Another big concern is with the '7-meter blackout'. By artificially loweing your CO2 concentration, you fool your body to 'sense' that no O2 needed, but when you rise from the depths, the negative pressure gradient, makes the O2 diffusion more difficult... and... guess what?? you blackout :|. So you risk your life in two different ways. Another plus is that your HR rises, so the dive is very unpeaceful.
What most people do is just make a few purge breaths (fast, deep exhales) to
release a litle CO2 for 'comfort dive' and then inhale and go. Or purge till you feel a litle light headness, then take 3 or 4 normal (as in normal life) breaths till the feeling goes away, big inhale and go
The number of purges is completely personal, from 0 to as many as 10, but they are typically 3-4. It depends on so many factors.. and same person different days can have different blood PH which turn into more/less purge breaths...
But hey, the good news is that if you already know the feeling of hyperventilation,
you just know that if you feel it, you must wait a litle breathing normally till the feeling disappears and then go in a SAFE way!!
SanSan:
When you have your torso/part of your torso inmersed in water, your breathing muscles are going to work much harder for the same amount of aire inhaled/exhaled that in dryland. The reason? water is much more dense than air, so forces to move it are higher. So maybe your 4-5 medium breaths dry are 9-10 in water (due to decreased volume) or your 4-5 medium dry breaths must be 4-5 wet much deeper (objectively same volume, but subjectively much hard to do) breaths.
This is the reason because swimming is so good for your lungs: you are breathing against the water
Cheers!