This section is from the book "Mahanirvana Tantra (Tantra of the Great Liberation)", by Translated by Arthur Avalon (Sir John Woodroffe). Also available from Amazon: Mahanirvana Tantra: Tantra of the Great Liberation.
This word, which comes from the root "to place," means placing the tips of the fingers and palm of the right hand on various parts of the body, accompanied by particular mantra. The nyasa are of various kinds. Jiva-nyasa follows upon bhuta-shuddhi. After the purification of the old, and the formation of the celestial body, the sadhaka proceeds by jiva-nyasa to infuse the body with the life of the Devi. Placing his hand on his heart, he says the "so-hang" mantra ("I am He"), thereby identifying himself with the Devi. Then, placing the eight Kula-kundalini in their several places he says the following mantra: Ang, Kring, Kring, Yang, Rang, Lang, Vang, Shang, Shang, Sang, Hong, Haung, Hangsah: the vital airs of the highly blessed and auspicious Primordial Kalika are here. "Ang, etc., the embodied spirit of the highly blessed and auspicious Kalika is placed here." "Ang, etc., here are all the senses of the highly auspicious and blessed Kalika," and, lastly, "Ang, etc., may the speech, mind, sight, hearing, smell, and vital airs of the highly blessed and auspicious Kalika coming here always abide here in peace and happiness Svaha." The sadhaka then becomes devata-maya. After having thus dissolved the sinful body, made a new Deva body, and infused it with the life of the Devi, he proceeds to matrika-nyasa. Mahika are the fifty letters of the Sanskrit alphabet; for as from a mother comes birth, so from matrika, or sound, the world proceeds. Shabda-brahman, the "Sound," "Logos," or "Word," is the Creator of the worlds of name and of form.
The bodies of the Devata are composed of the fifty matrika. The sadhaka, therefore, first sets mentally (antar-matrika-nyasa) in their several places in the six chakra, and then externally by physical action (Vahy-amatrika-nyasa) the letters of the alphabet which form the different parts of the body of the Devata, which is thus built up in the sadhaka himself. He places his hand on different parts of his body, uttering distinctly at the same time the appropriate matrika for that part.
The mental disposition in the chakra is as follows: In the Ajna Lotus, Hang, Kshang (each letter in this and the succeeding cases is said, followed by the mantra namah); in the Vishuddha Lotus Ang, Ang, and the rest of the vowels; in the Anahata Lotus kang, khang to thang; in the Manipura Lotus, dang dhang, etc., to Phang; in the Svadisthana Lotus bang, bhang to lang; and, lastly, in the Muladhara Lotus, vang, shang, shang, sang. The external disposition then follows. The vowels in their order with anusvara and visarga are placed on the forehead, face, right and left eye, right and left ear, right and left nostril, right and left cheek, upper and lower lip, upper and lower teeth, head, and hollow of the mouth. The consonants kang to vang are placed on base of right arm and the elbow, wrist, base and tips of fingers, left arm, right and left leg, right and left side, back, navel, belly, heart, right and left shoulder, space between the shoulders (kakuda), and then from the heart to the right palm shang is placed; and from the heart to the left palm the (second) shang; from the heart to the right foot, sang; from the heart to the left foot, hang; and, lastly, from the heart to the belly, and from the heart to the mouth, kshang. In each case ong is said at the beginning and namah at the end. According to the Tantra-sara, matrika-nyasa is also classified into four kinds, performed with different aims, viz.: kevala where the matrika is pronounced without vindu; vindu-sangyuta with vindu; sangsarga with visarga; and sobhya with visarga and vindu.
Rishi-nyasa then follows for the attainment of the chatur-varga. The assignment of the mantra is to the head, mouth, heart, anus, the two feet, and all the body generally. The mantra commonly employed are: "In the head, salutation to the Rishi (Revealer) Brahma; in the mouth, salutation to the mantra Gayatri, in the heart, salutation to the Devi Mother Sarasvati; in the hidden part, salutation to the vija, the consonants; salutation to the shakti, the vowels in the feet, salutation to visargah, the kilaka in the whole body." Another form in which the vija employed is that of the Aiya: it is referred to but not given in Chap. V., verse 123, and is: "In the head, salutation to Brahma and the Brahmarshis, in the mouth, salutation to Gayatri and the other forms of verse; in the heart, salutation to the primordial Devata Kali, in the hidden part, salutation to the vija, kring; in the two feet, salutation to the shakti, Hring; in all the body, salutation to the Kalika Shring."
Then follows anga-nyasa and kara-nyasa. These are both forms of shad-anga-nyasa. When shad-anga-nyasa is performed on the body, it is called hridayadi-shad-anga-nyasa; and when done with the five fingers and palms of the hands only, angushthadi-shad-anga-nyasa. The former kind is done as follows: The short vowel a, the consonants of the ka-varga group, and the long vowel a, are recited with "hridayaya namah" (namah salutation to the heart). The short vowel i, the consonants of the cha-varga group, and the long vowel i, are said with "shirasi svaha" (svaha to the head). The hard ta-varga consonants set between the two vowels u are recited with "shikhayai vashat" (vashat to the crown lock); similarly the soft ta-varga between the vowels e and ai are said with "kavachaya hung." The short vowel o, the pavarga, and the long vowel o are recited with netra-trayaya vaushat (vaushat to the three eyes). Lastly, between vindu and visargah the consonants ya to ksha with "kara-tala-prishthabhyang astraya phat" (phat to the front and back of the palm).
The mantras of shadanga-nyasa on the body are used for Kara-nyasa, in which they are assigned to the thumbs, the "threatening" or index fingers, the middle fingers, the fourth, little fingers, and the front and back of the palm.
These actions on the body, fingers, and palms also stimulate the nerve centres and nerves therein.
In pitha-nyasa the pitha are established in place of the matrika. The pitha, in their ordinary sense, are Kama-rupa and the other places, a list of which is given in the Yogini-hridaya.
For the attainment of that state in which the sadhaka feels that the bhava (nature, disposition) of the Devata has come upon him nyasa is a great auxiliary. It is, as it were, the wearing of jewels on different parts of the body. The vija of the Devata are the jewels which the sudkaka places on the different parts of his body. By nyasa he places his Abhishta-devata in such parts, and by vyapaka-nyasa he spreads Its presence throughout himself. He becomes permeated by it losing himself in the divine Self.
Nyasa is also of use in effecting the proper distribution of the shaktis of the human frame in their proper positions so as to avoid the production of discord and distraction in worship. Nyasa as well as Asana are necessary for the production of the desired state of mind and of chitta-shuddhi (its purification). "Das denken ist der mass der Dinge." Transformation of thought is Transformation of being. This is the essential principle and rational basis of all this and similar Tantrik sadhana.