The method doesn't come without risk. The breathing technique reduces levels of CO2 and ups the pH in the blood, a shift that can cause confusion, dizziness, and fainting.
Frigid water often prompts an involuntary gasp, a move which can cause people to drown. Cold plunges also trigger the body's fight or flight response, and in some cases, can induce "cold shock" and heart arrhythmias.
Some small studies show the method enables people to voluntarily influence their autonomic nervous system and immune system -- bodily functions previously thought out of reach of the human mind.
But more randomized-controlled trials are needed before we can understand how the method impacts the mind and body.