I just looked through quite some documentation, but couldn't find it that quickly (when I do, I'll update this). I use that graph in my own course materials.
It origins from study by Gavin/Mitchell where the end-tidal CO2 levels were measured in CCR diving. The Work-Of-Breathing is slightly higher on closed-circuit than open-circuit, but the physiology of the lungs and trachea doesn't change.
Our body has evolved to effectively breathe at 1 atmosphere. The gasdensity is about 1.23 grams per liter at sea level in 15°C.
Once you descend, pressure increases and so does the gas density. And with increased pressure our physiology doesn't work flawless anymore. It becomes harder to breathe out, since the air is much denser. You're not breathing out as effective as on the surface. Because there's more residual CO2 in your lungs, the blood can off-gas less CO2 to the lungs.
On the surface, your brain stem would immediately send a signal to your lungs to breathe faster. Under pressure, that mechanism doesn't work at 100%. The urge to breathe faster is diminished and the CO2 levels keep rising.
It gets really problematic when the gas density exceeds 5.8 g/l (at 40m/130ft the air density is almost 6 g/l !!)
Anything that turns oxygen into carbondioxide adds to a CO2 hit. Your brain is the biggest oxygen consumer.
Add some excess muscle movements to this, like chasing a weightpocket down to 55m, grabbing it and then trying to ascend with strong fin kicks, and the requirements for a brain-shutdown have been met.
You simply fall asleep like a switch turns off the light.
