Patanjali yoga sutras says ”
“Prachchardana vidharanabhyam va pranasya”
“By splitting and retention of breath in a special manner.”
“Vishayavati va pravruttirutpanna manasaha sthithinibandhini”
“By that very activity in the mind, through that very objects of senses, the mind can be made steady, brought to a standstill.”
Breaking the rhythms of the breath, the natural rhythms of the breath and holding the breath, sustaining the breath in different rhythms. If someone asks you where has Patanjali talked about (Sudarshan) kriya, Patanjali has not mentioned Sudarshan Kriya directly. But in this one sentence there is a clue. You can have the trace of our practices to this one sūtra. We are doing the same thing, the rhythms. We are not breathing just however the breath goes, but we are bringing it consciously in a definite, particular rhythm. That is prachchardana, splitting. Whatever rhythm the breath is coming through, splitting it and holding it on to different rhythms.
Suppose this does not work. This does work but if you feel that there still a little incompletion, then vishayavati means, through any one object of senses, the mind can be stilled. After singing bhajans, when we sit quiet. Then what is happening? Your mind is not running here and there at that time. Then you sit with your eyes closed and the mind is completely settled. Meditation. Mind has come to that stand still, hasn’t it?
Source : http://www.artofliving.org
Scientifically, Sudarshan Kriya improves antioxidant status at the enzyme and the gene level and reduce DNA damage and cell aging.
Every emotion has breathing patterns; for example, if one gets angry then one’s breath is short and fast. Similarly, breathing has an impact on the mind (for example, a deep, long breath relaxes the mind.) Through breathing in different patterns, one can influence emotions and gain control over one’s mind. Research suggested that it helps in improving well being (improving immunity and detoxification),and peace of mind (reduce stress, enhance brain function). Sudarshan Kriya is taught in various Art of Living programs.
A research published observed that a comprehensive practice of Yoga, Sudarshan Kriya & Pranayam has rapid and significant effect on gene expression suggesting long term beneficial effects even at the molecular level.
Source: Wikipedia.com
Sudarshan Kriya - Benefits and Experiences 