The Upanishad tells when to assume sanyasa and also the features of a sanyasi. It describes the dialogue between King Janaka and Sage Yajnavalkya.
King Janaka of the Videhas respectfully approached the sage Yajnavalkya and said: 'Revered Sir, expound to me renunciation'. Yajnavalkya said: Having completed the stage of a celibate student one may become a house-holder. From the stage of the house-holder he may become a forest-dweller (Vanaprastha) and then renounce. Or else he may become a mendicant monk from the stage of a celibate student or a house-holder or a forest-dweller. (There is also the provision that) a person may renounce worldly life that very day on which distaste for it dawns on him, whether he is not observing vows (before the stage of renunciation) or observes them, whether he has undergone the prescribed ablution on completing the disciplined studentship or not, whether he is one who has discontinued maintaining ritual fire at the death of his wife (Utsannagni) or one who does not maintain the ritual fire (anagnika).
Possessing a form as one new-born (i.e. unclad) unaffected by pairs (of opposites, such as heat and cold, pleasure and pain); accepting nothing (except alms, for bare sustenance); well established in the path of the truth of Brahman; of pure mind; receiving alms into the mouth (lit. into the vessel of the belly) at the prescribed time in order to sustain life, becoming equanimous at gain and loss (of alms), drinking water from the vessel of hand or from a water vessel, begging alms but to store in the belly; devoid of any other vessel; the watering place serving as water vessel; sheltering, equanimous at gain and loss of it, in an abode which is free from disturbance and is solitary (such as) an unoccupied house, a temple, a clump of (tall) grass (or hay stack), an ant-hill, the shade of a tree a potter's hut, a hut where ritual fire is kept, the sandy bank of a river, a mountain thicket, a cave, a hollow in a tree, the vicinity of a water fall or a piece of clean ground, without residing in a fixed abode; making no efforts (for gainful activity) and deeply intent on the uprooting of good and bad actions - such a sage who finally gives up his body in the state of renunciation is indeed a Paramahamsa.
Satyayani Upanishad is one of the Sannyasa Upanishads that belongs to the Shukla Yajurveda. This Upanishad says that the sage's inner possession is Om (the Pranava) and urges not to abandon it until his final breath. It explains that those who possess Om possess everything. The Upanishad warns that a saint who abandons this asceticism which is the highest spiritual life, is a great sinner. Abandoning all other stages of life, a wise man should strive for liberation. Having embraced asceticism, if one does not remain observing its laws, he is to be known as 'fallen from grace' (arudhachyuta).
This describes the way and detailed method to chant the holy letter "OM', which is called here as the nectar like letter. The wise, having studied the Shastras and reflected on them again and again and having come to know Brahman, should abandon them all like a fire-brand. Pratyahara (subjugation of the senses), Dhyana (contemplation), Pranayama (control of breath), Dharana (concentration), Tarka and Samadhi are said to be the six parts of Yoga. Just as the impurities of mountain-minerals are burnt by the blower, so the stains committed by the organs are burned by checking Prana.
Through Pranayamas should be burnt the stains; through Dharana, the sins; through Pratyahara, the (bad) associations; and through Dhyana, the godless qualities. Having destroyed the sins, one should think of Ruchira (the shining). Ruchira (cessation), expiration and inspiration - these three are Pranayama of (Rechaka, Puraka and Kumbhaka) expiration, inspiration and cessation of breath. That is called (one) Pranayama when one repeats with a prolonged (or elongated) breath three times the Gayatri with its Vyahritis and Pranava (before it) along with the Siras (the head) joining after it. Raising up the Vayu from the Akasa (region, viz., the heart) and making the body void (of Vayu) and empty and uniting (the soul) to the state of void, is called Rechaka (expiration).
That is called Puraka (inspiration) when one takes in Vayu, as a man would take water into his mouth through the lotus-stalk. That is called Kumbhaka (cessation of breath) when there is no expiration or inspiration and the body is motionless, remaining still in one state. Then he sees forms like the blind, hears sounds like the deaf and sees the body like wood. This is the characteristic of one that has attained much quiescence.
That is called Dharana when the wise man regards the mind as Sankalpa and merging Sankalpa into Atman, contemplates upon his Atman (alone).
That is called Tarka when one makes inference which does not conflict with the Vedas. That is called Samadhi in which one, on attaining it, thinks (all) equal. Seating himself on the ground on a seat of Kusa grass which is pleasant and devoid of all evils, having protected himself mentally (from all evil influences), assuming either Padma, Svastika, or Bhadra posture or any other which can be practised easily, facing the north and closing the nostril with the thumb, one should inspire through the other nostril and retain breath inside and preserve the Agni (fire). Then he should think of the sound (Om) alone.
Om, the one letter is Brahman; Om should not be breathed out. Through this divine mantra (Om), it should be done many times to rid himself of impurity. That which never decays is Akshara (Om) which is without Ghosha (third, fourth and fifth letters from 'K'), consonant, vowel, palatal, guttural, nasal, letter 'R' and sibilants. Prana travels through (or goes by) that path through which this Akshara (Om) goes. Therefore it should be practised daily, in order to pass along that (course).
Prana is thirty digits long. Such is the position (of range) of Pranas. That is called Prana which is the seat of the external Pranas. Of the Pranas the first viz., Prana is pervading the heart; Apana, the anus; Samana, the navel; Udana, the throat; and Vyana, all parts of the body. That man is never reborn wherever he may die, whose breath goes out of the head after piercing through this Mandala (of the pineal gland). That man is never reborn.
It gives the knowledge of athma in the tasty form of nectar and gives it as briefly as a dot. It tells us about what is Brahman and other concepts of hard core Hindu Philosophy. The mind is chiefly spoken of as of two kinds, pure and impure. The impure mind is that which is possessed of desire, and the pure is that which is devoid of desire.
It is indeed the mind that is the cause of men's bondage and liberation. The mind that is attached to sense-objects leads to bondage, while dissociated from sense-objects it tends to lead to liberation. So they think.
Since liberation is predicated of the mind devoid of desire for sense-objects, therefore, the mind should always be made free of such desire, by the seeker after liberation. When the mind, with its attachment for sense-objects annihilated, is fully controlled within the heart and thus realises its own essence, then that Supreme State is gained. The mind should be controlled to that extent in which it gets merged in the heart. This is Jnana (realisation) and this is Dhyana (meditation) also, all else is argumentation and verbiage.