"Why slowly exhale?" - So you don't increase the pressure in your lungs.
"Why is hyperventilating first bad?" - Should you hyperventilate and then hold your breath and any pressure is put on your lungs, YOU WILL PASS OUT! Don't do this!
Essentially hyperventilation reduces the blood's CO2 level. It becomes so low (while hyperventilating) that the body does not detect when the CO2 level subsequently rises to a dangerous level(while diving/holding your breath). The body fails to urge you to breathe fresh air and so you unwittingly pass out.
Deep-water divers hyperventilate and then take a big breath immediately before they dive. Sometimes "shallow water blackout" (same as what happens to kids in the "choking game) occurs, endangering the diver:
"Why is hyperventilating first bad?" - Should you hyperventilate and then hold your breath and any pressure is put on your lungs, YOU WILL PASS OUT! Don't do this!
Essentially hyperventilation reduces the blood's CO2 level. It becomes so low (while hyperventilating) that the body does not detect when the CO2 level subsequently rises to a dangerous level(while diving/holding your breath). The body fails to urge you to breathe fresh air and so you unwittingly pass out.
See the following two links for more info:
"The Choking Game: Self-induced hypocapnia": https://en.wikipedia.org/wiki/Choking_game#Self-induced_hypo...
Deep-water divers hyperventilate and then take a big breath immediately before they dive. Sometimes "shallow water blackout" (same as what happens to kids in the "choking game) occurs, endangering the diver:
https://en.wikipedia.org/wiki/Shallow_water_blackout