The art and science of Nyasa:

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 a 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 (MOTHER DIVINE). Placing his hand on his heart, he says the "so’hang" mantra ("I am That"), thereby identifying himself with the Devi. Then, the sadhaka places the eight Kula-kundalini in their several places. 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. Matrika are 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 (also Mantra Purusha) 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 (Bahiya-matrika-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. 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 through the body generally.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 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 bija 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). Transformation of thought is Transformation of being. This is the essential principle and rational basis of all this and similar Tantrik sadhana.
Nyasa is a powerful and important practice taught for advanced aspirants seeking to attain the special favors and support of the Devi in attaining the goal of her/his sadhana. Nyasa is the method of invoking the forces of Divinity in the body-mindfield of the aspirant thus making the aspirant's body-mindfield a walking Mantra Purusha. This also helps shield from negative energy vibrations and acts as a "kavacha" or a shield of Divine Protection and Preservation.