The above is a question about the "Why" of Laghunyasam. The time, precautions and reasons. However my question is not the same and I ask for what Laghunyasam symbolizes and encompasses. What energy does it invoke?
Nyasa was already explained here.
The following explanation on Mahanyasa and Laghunyasa can be found here.
FIVE FACES OF SHIVA:-
Because of his five faces Shiva has the name pañcavaktraḥ। pañcasaṅkhyākāni vaktrāṇi mukhāni nānārūpāṇi yasya sa tathā । (Shivasahasranamam-Padmapuranam).
These faces have the following names. īśāna, tatpuruṣa, aghora, vāmadeva, and sadyojāta. These faces face up the sky, East, South, North and West respectively. It seems that a lingam with five faces as described here is to be seen only in the Pashupati temple in Nepal and the temple attached to the Shankara math in Tiruvanaikkaval near Trichy. The hymn given in the beginning has salutations to the five faces of the Lord.
We have the salutations to Shiva's five faces in Mahanyasam that is chanted before chanting the vedic hymn Shri Rudram. There are two Nyasams, the Mahanyasam and the Laghunyasam.
Maha(mahat) means big. Laghu is the opposite,(laghu) means light, not heavy. The Mahanyasam is a part of the methodology of chanting of the Rudram on special occasions such as Pradosham, Shivaratri.
Typically, during a Rudra parayanam at home, one recites the Laghunyasam.
Such a Shri Rudra parayanam in a public place of worship such as a temple is preceeded by the recital of the Mahanyasam.
As the name difference indicates, the Laghunyasam is shorter.
The Mahanyasam invokes the deity 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 immediate presence of the Lord in the vicinity of the chanting. The Mahanyasam has certain chapters of the Shri Rudram, the Purusha Suktam etc repeated in it. The Shiva samkalpam is also part of this.