Yoga Sutras of Patanjali translated by Yogi Kalinath - Chapter 3, Verse 21
Pada 3, Sutra 21
Kaaya-roopa-sanyamaat tad-graahya-shakti-stambhe caksuh-prakaashaamprayoge ‘ntardhaanam
By constraint on the form of the body, upon suspension of the ability to be seen, that is disrupting the light emanating from the body to the eye, there arises invisibility.
An object can only be perceived by the brain once light reflects off the object which is then captured by the apparatus of seeing, the eye. Therefore if you perform constraint upon the visibility of your body and prevent the visible light to reach the eyes of another, you will become invisible to them.
This type of constraint can also be applied to the various senses such as hearing, smelling, touching and even tasting where one will be able to not be heard, smelled, touched or even made not to taste food.
All applications of the mind have their more subtle and deeper connections. In this sense, it is the development of humility which comes from the silence and self-effacement of the Spirit Within - making the ego less visible.