Just piggybacking on this a bit with some tips in case anyone reads this comment and wants to try it:
Do this laying down. You are essentially hyperventilating. By the 35th cycle, you should be feeling very lightheaded, possibly seeing stars. You absolutely can pass out or at least fall down during this process. Do it laying down or at least sitting so you don't injure yourself. Obviously, definitely don't do this in the water (you don't do the breathing exercises and ice baths at the same time).
The inhale through your nose you do fully, really trying to get as much air into your lungs as possible. The exhale that follows is really just a matter of relaxing everything and letting your ribcage collapse back down and the air leave; you shouldn't be pushing it out.
If you've never done it before, it can be fun to have a clock or timer going where you can see it. See how long you can comfortably hold your breath before that urge to inhale comes. You'll be amazed. Similar techniques to this breathing exercise (hyperventilating to lower the carbon dioxide levels in your blood) are one of the ways magicians like Houdini and others that do underwater escapes can hold their breath for so long (of course, don't try that at home).
Do this laying down. You are essentially hyperventilating. By the 35th cycle, you should be feeling very lightheaded, possibly seeing stars. You absolutely can pass out or at least fall down during this process. Do it laying down or at least sitting so you don't injure yourself. Obviously, definitely don't do this in the water (you don't do the breathing exercises and ice baths at the same time).
The inhale through your nose you do fully, really trying to get as much air into your lungs as possible. The exhale that follows is really just a matter of relaxing everything and letting your ribcage collapse back down and the air leave; you shouldn't be pushing it out.
If you've never done it before, it can be fun to have a clock or timer going where you can see it. See how long you can comfortably hold your breath before that urge to inhale comes. You'll be amazed. Similar techniques to this breathing exercise (hyperventilating to lower the carbon dioxide levels in your blood) are one of the ways magicians like Houdini and others that do underwater escapes can hold their breath for so long (of course, don't try that at home).