Every second you hold your breath, oxygen begins to drop. If the oxygen in the brain drops below a certain level you black out.
It starts with visual disorientation and light hallucinations, then the fingers, toes, hands, and feet start to tingle. You lose muscle control. These symptoms progress until you enter a state of spacy euphoria accompanied by wildly colorful dreams, the aforementioned pink cloud.
A person can stay in this blacked-out state safely for about two minutes until brain oxygen gets so low that you enter what’s called an anoxic state. Anoxia will trigger the body to initiate a last-ditch effort to breathe, called a terminal gasp. If there is no oxygen available at this time (for instance, if you’re underwater), brain damage begins to occur and you’ll eventually die.
-James Nestor, Deep