Shallow water blackout is more from hyperventilating (more than say 2 deep breathes) before free diving
(for scallops!) which lowers your blood pCO2 (partial pressure of carbon dioxide) As it is the increasing pCO2 that gives you the urge to breathe, you will have just delayed that urge, while at the same time using more oxygen. You will now have a low pO2. You will blackout from this lack of oxygen before feeling the urge to breath (from rising pCO2). Therefore you (may) drown.
On your DIY supply of air pulled down to say 2 metres. This will be now less in volume and also higher in pressure (equal to the surrounding water pressure). If you take a big breathe of this compressed air, and just maybe something goes wrong, and just maybe, because you haven't practised, you rush (or slowly, doesn't matter) to the surface, without opening your throat, your lungs will increase in volume by the ratio of air pressure at 2metres to pressure at sea level. As the pressure gradient is greatest in the first few metres, guess what, you may 'have a fatal incident'; your lungs will rupture. Seriously. It is apparently not hard to keep you throat closed against the increasing pressure from air in your lungs. Be careful.
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