One of the Shabbat prohibitions enacted by Hazal is that of Hatmana, which forbids insulating hot food before Shabbat in a manner that adds more heat to the food. If the insulation not only retains the pot’s heat, but is "Mosif Haval" – increases its heat – then one may not insulate the pot in this manner before Shabbat. In ancient times, people would keep food warm by insulting the pot in hot coals, which added to the pot’s heat so the food can continue cooking. The Sages were concerned that if one insulated a pot in coals before Shabbat, which clearly indicates that he wants the food to continue cooking, he might stoke the coals on Shabbat, which would constitute a Torah violation. They therefore enacted a provision forbidding insulating food before Shabbat in a manner that is "Mosif Haval."
The Shulhan Aruch applies this prohibition even to "Hatmana Mi’sad Ehad" – insulating just one side of the pot. Meaning, according to the Shulhan Aruch’s ruling, it is forbidden even to place a pot on top of hot coals, such that only the bottom is insulated. Since this type of arrangement also increases the pot’s heat, it is forbidden, despite the fact that the majority of the pot is not insulated.
In light of the Shulhan Aruch’s ruling, one might wonder why it is customary to place hot food on a "blech" – a metal sheet over the fire – before Shabbat. The blech with the fire underneath certainly increases the pot’s heat, and thus this should seemingly be forbidden due to the prohibition of Hatmana. The answer is that "Hatmana Mi’sad Ehad" is forbidden only because one might eventually thrust the entire pot into the coals, and might then stoke the coals. When it comes to a hard, solid surface, such as a "blech," there is obviously no possibility of thrusting the entire pot into the heat source. Therefore, it is entirely permissible to place a pot of cooked food on a "blech" over a fire before Shabbat, and this is not included in the prohibition of "Hatmana Mi’sad Ehad." This is the ruling of several leading Halachic authorities, including Rav Shlomo Zalman Auerbach (Minhat Shelomo, 3:12), Rav Yosef Shalom Elyashiv, and Rabbi Moshe Halevi (Tefila Le’Moshe, 1:1).
Summary: Although it is forbidden to insulate food before Shabbat in a manner that adds to its heat, it is permissible to place cooked food on a "blech" over the stove before Shabbat, as is customary.
