The diaphragm is a large muscle sitting just below the ribcage in the body that controls our breathing, it controls the rate of breath, the depth of breath and the volume of breath as we inhale and exhale but we may be consistently using it wrong or sometimes in the worse case, not using it all. When the diaphragm is not being used, like any muscle, it atrophies, making it weaker, smaller and less usable in our daily practice of 20,000 breaths per day. Conscious thought or intention to the breath takes over autonomic control and allows us to sit in the driver’s seat of our bodies largest command centre. An incredible design by nature to give us access to our autonomic nervous system (ANS), the system that controls both our sympathetic nervous system (SNS) and parasympathetic nervous system (PNS) or simply our flight or fight & rest and digest responses. More information on this topic here https://www.theprivatesea.com.au/post/stress-management. Taking control of these systems at will is the first step, but being able to use the diaphragm to help activate or gain benefit from them is another
Mouth Breathing is not something anyone should be recommending bar one reason. We are stuck in bad patterns of short and shallow breaths, not activating the diaphragm fully or engaging the belly to lengthen the inhalation and exhalation. As long term mouth breathers (due to evolutionary dysevolution*) we now don’t have the airways large enough to truly retrain the diaphragm through deep bellowing breaths through the nose, this is why we need mouth breathing, specifically only for short periods of training.
Nasal breathing is natures intended way for most human and non-human animals to breathe. Picture a horse at full gallop, a leopard sprinting through the shrubs for its prey or a chilled out koala chewing on some eucalyptus in the trees, all are nasal breathing for the insurmountable benefits that breathing through the nose holds (Dogs or other animals that pant is the only exception to mouth breathing in the animal kingdom as they use it to thermoregulate body temperature). Nasal breathing increases oxygen levels in the blood, produces nasal nitric oxide (nasalNO), activates the PNS, balances blood pH levels, humidifies and warms inhaled air reducing damage to the respiratory system, filters air from toxins and pollutants and can increase athletic performance.
Mouth breathing should only be used to train the diaphragm. Be conscious of how you are breathing throughout the day and bring awareness back to the nose whenever you can. Sitting for a few minutes a couple of times a day and using mouth breathing to activate your diaphragm to expand on the inhale, picturing the air fill your belly and contract on the exhale, trying to push all that air out of the belly. You may find it easier to rest your hands or a book on your belly to visually see the rise and fall of your diaphragm. This style of breath training will begin to strengthen your breathing muscles and naturally become an unconscious action, the next step is to train your nasal cavity to expand enough for the volume of air necessary to fill the belly. Breath with the belly and not with the shoulders, nose breathe whenever possible and train your diaphragm to soon realise that your energy levels will rise, mood will increase, stress lowers incredibly and most importantly your body will function at it's optimal rage.