naisargikaḥ - natural; prāṇa – the vital energy; sambandhaḥ - the association.
Universal existence is bound by the law of the divine. Every action that unfolds in this universe happens in a predetermined manner. For example, the sun rises at a particular time and sets at a particular time. Upaniṣad-s say that sun, moon, air, etc carry out their duties promptly, fearing for the Lord. If any one of the elements deviates from its assigned duty, the universe will annihilate in no time. As a part of the system, prāṇa infuses life energy to all the living beings, till the time of death. Body is able to function only because of prāṇa. Energy of prāṇa is different from the all powerful divine energy. Infusion of prāṇa into gross body is an automated system, whereas, the infusion of divine energy is a sought after system. Prāṇa is available in plenty in the atmosphere and the source of divine energy is beyond our comprehension and at the most can be said that it originates from the cosmos.
This aphorism points out that the infusion of prāṇa is a natural mechanism that happens in the body of the yogi. It is beyond his control. Even while breathing, the yogi stands connected to the Supreme consciousness. Such mechanisms are controlled by Śaktī, the power centre of the absolute autonomy of the Lord Śiva. The yogi continues to exist in his physical body as long as automated flow of prāṇa continues. Till the connection between his body and prāṇa exist, it is the duty of the yogi to keep his body in a perfect condition. The stoppage of prāṇa to his body happens at the predetermined time and his soul dissolves into the eternity, not to be born again. The message that one should stand connected to the Lord at all times, is the message conveyed through this aphorism. At all times is indicated by the world prāṇa.