Sins (pāpa) and their expiations (prāyaścitta) have been discussed in detail in many of the dharmaśāstras and purāṇas. Among the prāyaścittas often recommended, one class is known as ‘kṛcchras,’ those which can be performed with difficulty and cause a lot of hardship to the performer. (See KṚCCHRA for details.)
Atikṛcchra is a special expiation of this class. This penance, to be observed for twelve days, is said to purify one of all sins except the ones listed under ‘mahāpātakas.’
The person who is observing this penance should eat only one morsel of food for the first three days, that too in the mornings. For the next three days he can eat one morsel in the evenings only. During the next three days he is permitted to eat one morsel, provided it is got without asking for it. For the last three days, he should fast completely.
Some writers like Yājñavalkya, allow a handful of food instead of one morsel only.