How controlling your breath controls your mind
Self-regulation is the ability to adapt to what is going on around you. It is about using your mind and body effectively to manage what the world throws at you. The most basic of these mechanisms are outside your control to begin with. A simple example is your heart rate. It is very hard to change it by will alone. Some people who train long and hard using biofeedback are able to do it to a certain extent, but it is not easy. Controlling your breath is something that you can easily do; even though our breathing is such an essential activity for life.
You control your breath
If you look into it breathing is tightly regulated by things that we are not aware of. We have chemical sensors that measure oxygen, carbon dioxide and other chemicals in your blood. If there is anything amiss these sensors alert the brain and it immediately responds to adjust breathing without you knowing. However you can decide to breathe faster or slower; you can even hold your breath for long periods. This voluntary control has its limits and the involuntary controls can override it, but it you can get pretty lightheaded and even pass out if you choose to breathe too quickly (please don't do that!) What I'm trying to say is that breathing is a pretty basic function that is very largely under your control.
That is probably why some scientists have suggested that self-regulation can be altered by voluntarily controlling breathing (see 'sources' below). The benefits of this are pretty staggering as effective self-regulation has been linked to managing emotions, controlling when you get to sleep, and controlling your ability to stop automatically doing things you don't want to do (check out my sources at the bottom of the post). This includes stopping yourself from panicking when you are about to give that speech or sit that exam.
Fight or Flight vs Rest and Digest
Automatic responses in the body are regulated through the 'autonomous nervous system. This system has two basic components that kind of work against one another: the sympathetic nervous system and the parasympathetic nervous system. I'm going to call the sympathetic nervous system the 'Hype' and the parasympathetic nervous system the 'Chill' system as that is more or less what they do.
You have probably heard of the 'fight or flight' response. This is what happens when you perceive a threat of some sort around you (a tiger) and you get ready to run away or fight it. The problem is that in modern times the same response gets triggered by threats such as exams, taxes and other stresses of life. As you can see, you cannot really fight or run away from taxes, making the 'fight or flight' response useless. It is the Hype system that creates the 'fight or flight' response.
You may not have heard of the 'rest and digest' response, which is induced by the Chill system. There is a long nerve, called the vagus nerve, that connects your brain directly to your heart, guts and other organs. This nerve carries mostly Chill system fibers and it is a two way system. Among many other things it slows the heart and gets your gut going so you can digest while you relax. But, that is not all, as I said it is a two way system and slow belly breaths will stimulate this nerve, which turns on the Chill system.
People who struggle to turn off the Hype system struggle to get a response from their vagus nerve. This can be measured by how much heart rate responds to vagus nerve stimulation - something called heart rate variability. People with a low heart rate variability are not so in control of their responses - people with a high heart rate variability are in much more control. That is why heart rate variability is used by researchers as a way to measure how good we are are managing our emotions and coping with difficult situations. It is a measure of our adaptability and resilience.
How to control your mind
The good news is that if you start off having a low heart rate variability (which means that you have limited control over how you react to situations) you increase your heart rate variability with training. One of the best ways to train it is by doing sets of calm belly breathing. Calm breathing will work in the short term if you are anxious about a situation and you do it just before getting yourself into it (like using it right before an interview), but you will only get the real benefits by regularly practicing it. That is why our app Feel Stress Free recommends it very frequently as one of the daily activities.
If you want to know more about this and want to see where all the claims that I make here come from please read this excellent dissertation by Matthew Russell from Kentucky University. He does a very thorough review of how slow belly breathing works and then he carries out an experiment to confirm what a lot of the evidence from his review was pointing at. Definitely worth a read.