Prana is Unsteady
Prana is unsteady and fickle through each inhalation and exhalation. It unsteadily and inconsistently switches flow from from the right nostril (called the pingala nadi) to the left nostril (called the ida nadi). This movement of the prana between the Ida and pingala nadis is what gives rise to the world of reality and karma and as long as this movement continues the countless cycle of birth and death and entanglement with the external world will continue as an unending cycle of karma is formed.
However it is said that as long as suffering in the world shall continue, God shall keep reincarnating in this world to mitigate and end mankind’s suffering. So what is this world? Each time that the breath flows from the Ida to pingala it passes through the central channel known as the Sushumna nadi and unites with it and the same happens when it flows from the pingala to Ida nadi. This constant association or “yoga” of the breath with the Sushumna gives rise to “yug” or the external world and the period of yoga or unision of the breath with the Ida and the Sushumna and the pingala with the Sushumna is what gives the experience of Godhood. Yogis attempt to increase this interim time of the breath in the Sushumna through their yogic sadhanas. when this union of breath in the shushumna stablizes, one can experience peace and godhood. This is what is exactly meant by the emergence of God in every yug from the outer to the inner world of yoga.