The breath not only sustains your very existence, but it can also be effectively utilised to channel and regulate prana in the body.
'Prana' is the energy or life-force that pervades everything that exists, and 'yama' is the control or regulation of that life-force. If prana is not flowing freely through the body, or there are blockages in certain areas preventing it's free-flow, optimal health can become compromised on all levels i.e physical, mental, emotional and spiritual.
The breath can be used to balance your energetic state according to what is going on in your own being and in the environment around you.
Breathing through the left nostril has a calming, nurturing effect, and is associated with the moon (lunar), whilst breathing through the right nostril has a stimulative, energising effect, and is associated with the sun (solar).
Exhalation is associated with the traits of left nostril breathing (lunar), and inhalation is associated with the traits of right nostril breathing (solar). Therefore, if you are wanting to promote calmness and tranquility, you would inhale through the left nostril, and exhale out of the right. Or, if you are needing a stimulative, energising effect, you would do the opposite i.e inhale through the right nostril and exhale out of the left nostril. Or if more balance is required, alternate nostril breathing can be utilised i.e inhale left nostril, exhale right nostril; inhale right nostril, exhale left nostril.
However, it is important to just become comfortable and familiar with your natural breathing first, and then practise inhaling and exhaling fully with awareness, through both nostrils, before undertaking more controlled breathing techniques.
A good way to learn to breathe more fully is to feel as if you are breathing into your entire body, rather than just your lungs.
And it is not only your breathing that can be used as a means of channeling prana, it is also the environment you are in, the food you eat, and the company you keep, that can either promote or hinder it's free-flow. Being around nature, eating fresh, healthy food, and being around happy, well-balanced, respectful people, enhances positive, life-force resonance.