When the Yogī becomes one with Śiva, he gets superhuman powers and becomes capable of creating whatever he likes.
Śaktisandhāne śarīrotpattiḥ शक्तिसन्धाने शरीरोत्पत्तिः (sūtra I.19)
Śakti – the will power or Icchāśakti; sandhāne – uniting; śarīra – body; utpattiḥ - production.
When the Yogī is fully united with his Icchāśakti, as discussed in sūtra I.13, he becomes capable of creating any type of body.
The Yogī always stays attuned with Śiva, hence he is not different from Śiva. How he has become one with Śiva is explained in sūtra I.13. The Yogī stays united with Śiva by means of his icchāśakti or will power and by doing so, he realises the splendour of Śiva. Because of this the Yogī is able to create his own body in whatever form he likes. But there is a significant precondition. The Yogī’s awareness or consciousness should remain united with Divine Consciousness at all times, not intermittently. By default, Yogī refers to a person whose individual consciousness stands perpetually united with Śiva Consciousness. In this state, all dualities are dissolved. Only this Yogī can become a true spiritual master, as he can at will initiate his disciples and make them reach higher levels of spirituality by subtly working on their kuṇḍalinī. This Yogī represents all kinds of divine energies, as he is now attuned with Śiva. Such a Yogī need not spend long hours on prayers and meditations. His one round of breathing is capable of manifesting all his volitions. However, he should not have any personal ambitions and should be concerned only about his disciples and followers. This Yogī is mediocre, as he has other thoughts still manifesting in him, for example the thoughts of his disciples and followers. This means that his mind is not pervaded only by Divine Consciousness, but also by temporal consciousness. The ultimate stage of a Yogī is achieved only when he does not have any other thoughts except Śiva Consciousness. Śiva entirely pervades his mind with no traces any other impressions.
Spanda-Kārikā (III.1) says that Śiva brings forth the unfulfilled desires of the Yogī, who has not yet abandoned the desire of staying put in his body. Śiva does this by bringing about equilibrium between prāṇa and apāna through samāna śakti. Prāṇa and apāna are the two of the five vital breaths. Samāna used here does not mean one of the vital breaths, but used to mean equilibrium between prāṇa and apāna. When there is equilibrium between prāṇa and apāna, the suṣumna or the central canal of the spinal cord is activated. Śiva appears in the suṣumna during the dream state of the Yogī and reveals to him his desired object, the vision of Śiva.
This sūtra subtly conveys that such a Yogī develops certain siddhi-s (superhuman powers). When the Yogī is in his active state, he has to pray to Him for the objects that he desires. From the active state, the Yogī moves to his meditative state and reach the state of samādhi (intense concentration leading to absorption of his mind). In the intent state of samādhi his prāṇa and apāna cease to function and when he re-emerges from his trance, prāṇa and apāna are again activated. During the time between the cessation of prāṇa and apāna and restoration of prāṇa and apāna, his prayer has been answered. However, the Yogī should always stay connected with Him, without any disconnect. This is how the Yogī is able to produce to whatever he desires. This is the right way of initiating a disciple. The disciple is able to reap the benefits of initiation (reaching higher levels of spirituality) only if he is initiated this way and not otherwise.