In Gita 5:27, 28 Lord reveals a Meditation technique.
Shutting our all external objects, fixing the vision between the eyebrows, making the inward and the outward breaths move within the nostrils evenly, the sage, who has controlled the senses, mind and intellect, who is bent on liberation, who has cast away desire, fear and anger, ie ever liberated. (Gita 5:27-28)
Lord says ” Praanaapaanau samau krtvaa naasaabhyantaracaarinau,” Here the emphasis is prana “life-force breath” The life breath exhaled is “praana”, while the breath inhaled is “apaana”. The speed of the exhaled breath is fast, while the speed of inhaled breath is slow. In order to regulate the process of exhalation and inhalation so that both of these may take equal time, first the “apaana” breath is inhaled with the left nostril (shutting the right nostril with the thumb), then the praana breath is exhaled, through the left nostril.
This process of exhalation and inhalation, should take equal time. Through constant practice the flow of the praana and the apaana breath becomes regulated and even, gentle and subtle. When there is no sensation of air inside or outside the nostril and in the throat, it should be understood that the flow of the praana breath and the apaana breath has become regulated and even. When this flow becomes even (because of the aim of God Realization), the mind starts meditating on God. This control of the breathing process, has been described here, because it plays and important role in the path of meditation.