What are "Retzafim"?
Rashi: They are coals, like "uv'Yado Ritzpah" (Yeshayah 6:6).
Malbim: They are Reshafim (flames).
What is "Ugas Retzafim"?
Radak: It is a cake baked on coals.
Malbim: It was a cake of flames. It seemed to him that the flames were united in the form of an edible cake.
Did he eat the entire cake and drink all the water?
Radak: No, for the angel told him to eat a second time (verse 7), and it does not mention that more food or water was given to him.
Malbim: It was a cake of flames. He was commanded to eat it. This is a parable that his body will be purified from earth (the heaviest of the four elements), and be like creations composed of the light elements - fire, water and wind. This is like "Oseh Mal'achav Ruchos Mesharesav Esh Lohet" (Tehilim 104:4). Demons' bodies are composed of fire and wind or fire and water. Chazal and Mekubalim say that they eat, i.e. things similar to their composition. The cake hints to Nevu'ah, and the water to Torah. The Targum of "uv'Yado Ritzpah" (Yeshayah 6:6) is uv'Fumei Mamlal (there were words in his mouth). It says "Hoy Kol Tzamei Lechu la'Mayim" (ibid., 55:1) - until now, his food was a Divine spiritual matter.
