That was a scene from Mission Impossible: Fallout in 2018, “Lane hyperventilates before being submerged, giving more oxygen to the blood/brain than a single deep breath, allowing him to stay conscious longer”. As fake or true as that sounds, it’s correct to a certain degree.
TL;DR: Why you should know about this
That technique Lane used is called “Shallow Water Blackout”, and you MUST NEVER do this in a pool or for fun, only use this for emergencies, period! It tricks your body (more precisely the brain) into thinking that you can hold your breath longer, but in reality your body is not getting enough oxygen and you would drown. You have zero control with hyperventilating, just take a deep breath instead.
What is shallow water blackout
It is more so an occurrence rather than a technique as stated in the previous section, an underwater “faint” due to a lack of oxygen to the brain; or, more formally:
“Shallow Water Blackout results from hypoxia (low oxygen) to the brain. What triggers one to breathe in elevation of carbon dioxide (CO2), not low oxygen (O2). One basically ‘blacks out’ or faints in the water. For some, their lungs will take on water leading to drowning while others simply suffocate or die of other causes brought on by the breath-holding. Death can be a result of the prolonged breath-holding, even if not from so called “Shallow Water Blackout.” Breath-holding may stimulate genetic triggers leading to various causes of death.”
How it happens and how to avoid it
It’s referred to as a “silent killer”, and occurs in only two minutes, opposed to regular drowning, which takes six to eight minutes. That’s because your body has no control at all when you’re hyperventilating, you won’t even notice that you’re lacking oxygen and you’ll faint and drown even though you feel fine at that time.
Once again, NEVER do this for fun, whether it’s a competition or just playing with your friends and family. You have no control and will most certainly faint.