The legend of her origin stresses Matangi's association with leftover food, which is normally considered highly polluting. Indeed, she herself actually arises or emerges from Shiva and Parvati's table scraps. And the first thing she asks for is sustenance in the form of leftover food (uccishtha). Texts describing her worship specify that devotees should offer her uccishtha with their hands and mouths stained with leftover food; that is, worshippers should be in a state of pollution, having eaten and not washed. Since for Matangi worshippers make offering in a polluted state, she is known to have been offered a piece of clothing stained with the menstrual blood in order to win the boon of being able to attract someone. Menstrual blood is regarded in almost all Hindu texts and contexts as extremely polluting, and menstruating women are forbidden to enter temples or otherwise serve the deities. In the case of Matangi, these strict taboos are disregarded, indeed, are flaunted.
This description by Nitin Kumar, Executive Editor, Exotic India.
Kinsley, David. Tantric Visions of the Diveine Feminine. New Delhi: Motilal Banarsidass, 1997.