Diaphragmatic breathing is a form of breathing exercise meant to strengthen your diaphragm, a large dome-like muscle just under your lungs and above your abdomen. It can be done by placing one hand on your belly, and one hand on your chest; when slowly breathing in, allow your abdomen to expand out while keeping your chest still. When exhaling, allow both areas to relax.
When your diaphragm contracts, it is pulled downward into what was abdominal space. This allows the lower lobes of your lungs to expand downwards and outwards as well, taking in a greater volume of oxygen . This extra oxygen helps to increase your energy levels and decrease symptoms of fatigue and lethargy.
Your heart sits on top of the diaphragm, just slightly left of centre. When your diaphragm contracts, it pulls and moves downwards, which allows the ventricles (bottom portions) of the heart to elongate and drop further, while the upper heart stays held in place by the vascular tubes bringing blood in and out of it. This allows the ventricles to fill up with more blood to be pumped out, which can help to lower your heart rate and blood pressure.
The vagus nerves (left and right) are major parasympathetic nerves that give our body the calming feelings that can help us to relax. They start in the brainstem (specifically the medulla oblongata), come down both sides of the neck, wind around the esophagus and through a hole in your diaphragm (called the esophageal hiatus) into the abdomen. When the diaphragm contracts, it squeezes gently around the esophagus and vagus nerves and pulls down, stimulating the nerves to initiate their parasympathetic activity, and promoting those feelings of relaxation and calmness.
Contracting the diaphragm initiates intra-abdominal pressure as it moves down into abdominal space, which then initiates spinal stability through erector muscles contracting in your mid to lower back. A pattern of breathing into this region ensures that stable pressure is maintained through breathing cycles. This can help provide strength to the area and prevent low back pain.