The merging of the movements of breathing, controlling the gross elements, diverting attention from all objective senses and directing it towards the center of the movement of the breath, and removing your consciousness from the grip of the elementary field.
In this sutra the word nadisamhara means "merging the movements of breath." The word bhutajaya means "attaining control of the gross elements." When you have control of the gross elements, then, when you suffer, you do not suffer internally. If you become feverish, internally you do not suffer any pain. In the sutra, the word bhutakaivalya means "taking the mind away from gross objects of senses and directing it toward the center of the movement of the breath." The word bhutaprithaktvani means "freeing your consciousness from the grip of the elementary field." For example, if you have a painful leg, you separate your consciousness from the pain of that leg and you are without pain. These powers are what yogis experience.
The word bhutajaya means to achieve control of the five elements from earth to ether through contemplation.
This is explained in the Svacchanda Tantra in this way:
Whenever you want to control the wind in your body, you must, through contemplation, put your awareness on the big toe of your left foot. When there is insufficient fire in your body (less warmth in your body), you should meditate by putting your awareness in the center of the navel. When there is a lessening of the flesh in your body, you have to contemplate on earth while putting your awareness in the pit of your throat to increase that flesh. When there is a lack of water or you are flooded, then you must contemplate on water while putting your attention on the inner tongue (ghantika) just near the talu. To attain all the powers that you desire, you must contemplate on the element ether (akasa) while putting your attention on your head. (Svacchanda Tantra 7.299-300)
The meaning of the word bhutebhyah kaivalyam is to be free of the elements. How is this accomplished? You must draw back (pratyaharanam) your mind from the objective field of sensory pleasures and concentrate it on the center of the breath.
So it is said in the Svacchanda Tantra:
Direct your consciousness to the center of the navel and also direct your mind to that center, carrying it away from the organs of the senses. This fourth breathing practice (pranayama) is a calmed means of retention. (Svacchanda Tantra 7.297)
In this pranayama, there is no breathing out and in. This has already been explained. From the heart, you should direct your consciousness to the center of the naval and at the same time, direct your mind, which is lost in objective pleasures, towards this center.
In the present sutra, the words bhuta prithaktvanimean "just to carry your awareness away from the gross elements." How does this happen? It happens when one's consciousness is not influenced by the elementary field.
About this subject, it is said in the Svacchanda Tantra:
When you subside the force of all of the five gross elements, up to the stage of unmana (beyond samana), you become one with Siva. (Svacchanda Tantra 7.327)
~ Shiva Sutras, The Supreme Awakening ~