Have you ever been to an exercise class and the instructor has shouted out 'don't forget to breathe'? Don't forget to breathe! How is it possible that you would ever forget to breathe? Breathing is an involuntary process the body adapts in order to stay alive!
Think about it. If you were to lift a heavy weight, you would probably take a short, sharp inhale and hold your breath while lifting the object. Your body already has an intelligent approach to the activities you put it through, without your conscious involvement, or voluntary control of the breath. In fact, your body knows best in this scenario. If you are lifting something heavy you would need to brace yourself as you lift, which means holding your breath as you tighten your core - maybe making sounds like urrrrrggghhhh! Weightlifters know this as the Valsalva manoeuvre.
But there is more to it that simply bracing yourself. Holding your breath actually builds up more concentrated levels of carbon dioxide in your blood, which in turn allows the oxygen in your blood to be released to the tissues. So, by holding your breath you are actually supplying your tissues with more oxygen, not less. Without this holding, a heavy lifter will be put at risk of damaging their spine, and a person focusing intently on a task will likely perform worse in the task as they let their concentration drift with the breath. One thing that holding your breath guarantees is focus.
Yoga emphasizes nose breathing. If you are breathing through your mouth you are likely to be over-breathing. For newcomers to yoga, it is necessary to initially create an awareness of breathing patterns, the benefit of placing the body in various postures to open and expand the breath, and the containment of the breath by introducing particular locks.
Sometimes you may use the intercostal muscles (muscles between the ribs) more when breathing, expanding the ribs forcibly as you inhale, particularly in back extensions. Sometimes, you may be using your abdomen more, keeping the chest relatively still, expanding the abdomen as you inhale, drawing the pelvic floor up slightly to keep control, and then strongly engaging the pelvic floor upon exhalation. Some exhalations in some postures will naturally encourage you to pull the upper abdomen in and up as you exhale fully, particularly postures such as headstand or downward-facing dog.
Eventually, you may find that you can remove all force and control of the breath and let the body's position dictate how you breathe. The breath is not forced, in that case, but is a natural result of the position of the body and the type of movement it is put through.
Slowing down the breath, with a natural pause between breaths, could be described as the ultimate breath retention, without force or strain, building up tolerance of carbon dioxide levels, making you a more efficient breather and a fitter, healthier and happier individual.