Chinnamastika is believed be the incarnation of Mother Goddess or Goddess Shakti that appeared before the Devas or Gods who sought for a wife for Shiva. Hindu god Shiva led an ascetic life. He saw no sense in transitory pleasures of life. Devas realized that this would create an upheaval in the universe as there will be no creation if all the living being start to follow the path of Shiva. So they prayed to Goddess Shakti to help in solving the peculiar situation.
Goddess Shakti then appeared before the Devas as Chinnamastika. In this form, Mother Goddess was headless. She held a sword in one hand and in the other hand was her cut head. The head was drinking the blood that spurted from her cut head.
Chinnamastika is flanked by Jaya and Vijaya. In some painting it is two women that are on both the sides of the Goddess.
Mother Goddess stands over Kama (God of Love) and his consort Rati. This shows conquest of lust.
Symbolically her portrait shows that through the mastery of primal instincts one can sever the head of the ego. When this happens the Sushmana, Ida, and Pingala flow free - the three divine chakra that exists outside the body apart from the known seven in the body. The three spurts of blood show that by severing the head of the ego one can possibly enter in the three divine Chakras that exist outside of the body.
Chinnamastika agreed to charm Shiva and bring Him into worldly life. She then appeared as Sati and married Shiva.