Some activities in the human body like the beating of the heart are automatic though they may follow a set rhythm while some activities like the limbic system can be controlled. But breathing is unique as it is both automatic and can be controlled.
In the context of yagna (selfless action) and breathing, Krishna says, "Some offer prana (incoming breath) into apana (outgoing breath) and apana into prana as sacrifice; some are absorbed in pranayama by restraining the courses of prana and apana"(4.29).
The duration and depth of the breath indicate the state of mind. For example, when we are angry our breath automatically becomes fast and shallow. On the contrary, by making our breath slow and deep we can control our anger. This implies that by controlling the breath, one can control the mind which gave rise to many techniques of meditation and pranayama.
While explaining 112 meditation techniques to Parvathi, Lord Shiva mentions about 16 techniques that are purely based on breath. In the contemporary world, we have numerous meditation techniques based on observing and subsequently controlling the breath. It is essentially the art of observation and mastering this art is easy by engaging the ever wandering mind with incoming and outgoing breaths which will make us stable. This art can be subsequently used to observe thoughts and feelings as observation and feelings or thoughts or lust don't go hand in hand. Finally, this leads to what is known as 'observer becoming observed', akin to sacrificing sacrifice.
Pranayama literally means control of breath and is practised through various techniques like kapalbhati. Prana means life energy like flowering or sprouting which continuously flows through us. Pranayama is streamlining that energy for harmony or resonance for joyful living and the lack of this harmony is nothing but agitation, fear and tension.
Source - Daily World