Breath is life. It is fundamental to every action and holds within it divine potential and divine essence. Breathing is rechaka and puraka, meaning inhale and exhale. Every inhalation is our inspiration from god, and every exhalation is our surrender to god. Life begins with an inhale and life ends with an exhale. Within the breath there is the potential for everything.
The function of breath in the Ashtanga yoga system cannot be emphasised enough. Yoga is a breathing practice. In the words of Sri K. Patthabi Jois, if you are not breathing you are just bending, not doing yoga.
All breathing should be done through the nose (unless otherwise specified) as breathing through the mouth weakens the heart. The breath should be deep, smooth, and consistent. Our breath is the momentum behind our movement and our stability within the posture. Correct breath allows for every movement and posture to be gentle, precise, powerful and steady. Both the inhale and exhale should be even, meaning, the length and intensity should be the same. Breathing in this manner purifies and stabilises the body, mind, and nervous system.
In Ashtanga yoga we practice Ujjayi Pranayama, meaning, Victorious Breath. By slightly constricting the throat we create a deep hissing sound as we breathe. Ujjayi breath maintains the balance and homeostasis within the body during practice, cultivates and expands prana, circulates the blood, builds the internal heat of purification, and is the vehicle by which we can overcome obstacles and control our mind and body.