"Yoga" is a Sanskrit term, which connotes the correspondence of body, mouth, and mind. It is the gesturing of the hands in a mystical position (mudra) together with the recitation of esoteric words (dharani, mantra) and the concentration of the mind in contemplation. "Ulka-mukha" (flaming mouth) is represented in the Buddhist sutras as a hungry ghost (preta). In addition to a very thin appearance, it has a throat that is no bigger than needles, and a mouth that spits out flame.
According to the Ulka-mukha Preta Sutra, Ananda once saw the incarnation of Avalokitesvara Bodhisattva (goddess of mercy) as lord of hungry ghosts while practicing meditation in the forest. The Bodhisattva thus incarnated herself to save all suffering beings in the ghost realm. The lord was emaciated in appearance with hideous features. Its hair was unkempt; its nails and teeth were long and sharp. Its throat was needle-like; its stomach jutted out like a mountain, and flame spurted out of its face. Ananda was flabbergasted, and asked about the cause of such frightening destination. The lord told him that he was greedy and miserly while he was alive; thereby upon his death, he descended into the realm of hungry ghosts and transformed into the present semblance. He further had to endure all kinds of sufferings, and year-round starvation. Moreover, he informed Ananda that he, Ananda, would pass away in three days, and would likewise suffer the same destiny. Ananda was terrified; he hurriedly went to the Buddha to seek redemption. The Buddha explained The Discourse on the Feeding of Hungry Spirits and taught him the proper way of bestowing food. If living beings can give food and drink to the infinite number of hungry ghosts and deities, not only will they never descend into the realm of hungry ghosts, but will also acquire longevity. While being watched over by all spirits and gods, they will have good fortune in every endeavor.
The Yogacara Ulka-mukha Dharma service is held in accordance to the sutra. Although the service is performed to rid the hollow hunger of the ghosts by bestowing food and drink to them, more importantly, it is to deliver them from all sufferings through the teachings of Buddha. By listening to the Dharma, the ghosts will then take refuge in the Triple Gems, receive the precepts, and thus acquire Right View, which will enable them to abstain from bad deeds and the terrifying consequences therewith. Only then is enlightenment within their grasp. (Source: IBPS Vancouver)