I do two forms of breathing exercise that I have not been able to find any information about.
The first is to hyperventilate for 30-100 breaths. When I do this I feel a tingling in the center of my forehead and feel muscles relaxing in my head.
The second is to take as deep a breath as I can (only works while standing), then hold it in for as long as I can. When I do this, after I breathe out I get various effects. I feel energy flowing through my head and face. I feel my head and face relaxing. I sometimes get thoughts like "I am going to die," but I don't take them seriously. I feel like this is placing my nervous system under stress, resulting in a surge of energy that has various effects. The feelings I get from doing this exercise are very varied, I can't think of all of them off the top of my head. Words don't do justice to how profound these feelings are. I get enough out of doing this to keep me interested. I work at a standing desk, and do this exercise hundreds of times a day. Every time is interesting. I feel like I am tapping in to some sort of secret with this exercise.