Sage valmiki uses many metaphors and writes in a poetic sense while explaining an incident in the Valmiki Ramayana. There are hundreds of usages of Upama and rupaka alankara which is used to explain the similarity between two incidents or show the non difference between two incidents. For example, In Sundara kanda, Hanuma sees Ravana and the description is given like this:
That Hanuma also saw the wealthy Ravana's arms tied with golden armlets thrown apart resembling flag staffs raised in honour of Indra. Those arms had wounds made by stabbing from the pointed edges of tusks of Iraavata and fleshy shoulders smeared with diamond and hit by Vishnu chakra. These arms of Ravana were opulent with well grown and equal strong shoulders proportionate to body, with knees and thumbs of good characteristic, with good fingers and palms. With strong joints in the shape of club, round in shape resembling the trunks of an elephant like five headed snakes thrown on clean couch.
There are hundreds of examples more. Some examples are taken which are known naturally like "an agry snake hissing" to show person is anger or well known incidents like "just like Garuda snatches a snake". Similarly, I found an example Valmiki maharshi uses to describe Kausalya's lamentation towards Sri Rama and says the following about Kaikeyi: (emphasis mine)
Pātayitvā tu kaikeyyā rāmam sthānāt yathā iṣṭataḥ |
pradiṣṭaḥ rakṣasām bhāgaḥ parvaṇi iva āhita agninā || 2-43-5
"Having willfully thrown Rama out of his position, Kaikeyi has acted in the same way as one who tends the sacred fire throws a part of the sacrifice to ogres on a new or full moon." [Ayodhya Kanda Sarga 43 shloka 5]
I am asking about this bold part. Valmiki Maharshi generally uses references of incidents from Puranas or popular in the society of that time to describe something. But here, it is Kausalya speaking. Here, she says a sacrifice where a part is thrown for ogres (rakshasas) on new moon and full moon day. What are these sacrifices and why is a part is thrown for Rakshasas? What is the reference made by Kausalya here?