It is matter of common observation to note that stress makes us breathe fast and shallow. Conversely, we breathe deep and slow when we are relatively relaxed.
Babies breathe deep soon after they are born. They have relatively little stress. Anybody who sees a baby breathe while asleep can observe its abdomen rise on inhalation and fall on exhalation. This is the symptom of deep diaphragmatic breathing. Subsequent growing pains start to stress the infants replacing the natural rhythm of deep and slow breathing with fast and a shallow pattern.
It has been observed that by conscious and deliberate practice of deep breathing, we can learn to reduce our stress.
Physiologically, life is breathing in and breathing out. Psychologically life is stimulus in response out. Putting consciousness in the breathing process, puts consciousness in the stimulus response behaviour. Our psychological process mirrors the physiological process of life. Autonomic breathing is mirrored by the autonomic reactive behaviour whereas conscious mindful breathing is mirrored by conscious and mindful stimulus response behaviour.
Autonomic reactive behaviour is primarily driven by emotion and is hardly intelligent. On the contrary, mindful behaviour uses emotion intelligently. Conscious breathing therefore is a physiological way to develop emotional intelligence.
Stress is the creation of the ego. Management of stress with breathing in fact is the management of the ego. Stress results from the perception of a gap between expectations and reality. Conscious breathing thus has the effect of aligning expectations with the reality. Once behaviour is modified through practice, breath and consciousness returns to the original nature, moving autonomously as one, without a gap
Stress prevents insight. We are unable to perceive correctly, unable to get to the bottom of anything secular or spiritual. Spirituality is a matter of insight. Prevention of insight limits us only to the perception of the most apparent which the unseen is definitely not.
Making the ego extinct is merely a metaphor for making it ineffective in preventing insight.