Yoga Sutras 3.21
Patanjali, explained by Swami Vivekananda
कायरूपसंयमात्तद्ग्राह्यशक्ति-स्तम्भे चक्षुःप्रकाशासंप्रयोगेऽन्तर्धानम् ॥
kāyarūpasaṃyamāttadgrāhyaśakti-stambhe cakṣuḥprakāśāsaṃprayoge'ntardhānam
By making Samyama on the form of the body, the perceptibility of the form being obstructed and the power of manifestation in the eye being separated, the Yogi's body becomes unseen.
In plain words: By focusing deeply on the form of his own body, a yogi can block other people's eyes from seeing it. He does not truly vanish, but no one can see him.
Swami Vivekananda explains
A Yogi standing in the midst of this room can apparently vanish. He does not really vanish, but he will not be seen by anyone. The form and the body are, as it were, separated. You must remember that this can only be done when the Yogi has attained to that power of concentration when form and the thing formed have been separated. Then he makes a Samyama on that, and the power to perceive forms is obstructed, because the power of perceiving forms comes from the junction of form and the thing formed.