Mahanyasam (‘Maha’ meaning ‘Elabaorate/Great’ and ‘Nyasa’ meaning ‘Purification’), is recited to purify the body, mind and soul of all the impurities. It is a part of the methodology of chanting of the Sri Rudram on special occasions such as Ekadasa Rudram, Laghu Rudram, Maha Rudram and Ati Rudram. It invokes the diety of Shri Rudra in His many forms of Indra, Agni, Yama, Varuna etc in the different limbs (anga's) of the one who chants. Such an invoking is done to seek the blessings of the Lord, and at the same time giving one and all a feeling of the immediate presence of the Lord in the vicinity of the chanting. It has certain sections of the Sri Rudram, the Purusha Suktam etc repeated in it. The Shiva Sankalpam is also part of it. The self- purification process is mandatory prior to the chanting of Sri Rudram during the special occasions mentioned above because the Sruti is emphatic in stating "Na Rudro Rudra Marchayete", meaning that none can touch and worship the Siva Linga, which is sanctified, without one first becoming a Rudra. So Rudra's Kalas or features are invoked within the devotee to become a Rudra who can then commence the worship of the Siva Lingam. Anganyaas (nyasa involving Hriday, Siras, Sikha, Kavacha, Netra and Astra) and Karanyaas (nyasa involving fingers and palms of both hands) are the simplest forms of entering into any mantra sadhana. Mahanyasam is above all this ; it carriesthe Matrukas - the Beejaksharaas to all the parts of the body, which then are assumed as the parts of the Lord himself.
It must be remembered that the Mahanyasam confers on the person purity for that particular occasion only and not throughout the day and much less throughout one’s lifetime.
There are many versions of the Mahanyasam. What we learnt was the "Kumbakonam" (Ravana kruta) version which is the version published by the famous and revered "Anna" who has published so many wonderful easy-to-understand books on various pujas, homams and rituals.
Sources : various websites. https://grdiyers.weebly.com/mahanyasam.html
Please note that the links provided are to the best of our knowledge. If they are inaccurate please let DM group know.