Sauce from ĥametz food occasionally spills from pots onto the warming tray (“plata”), and since the plata is a heat source, the ĥametz is absorbed with the intensity of a kli rishon on the flame. On other occasions, dry ĥametz dishes (such as porridges, kugels, and quiches) fall onto the plata, which constitutes absorption through fire and must be koshered through libun.
A blekh (a metal sheet that is placed atop a gas range on Shabbat) can be koshered through libun if there is no alternative, even though it will warp and become damaged. But when it comes to an electric warming tray, libun is liable to melt the electrical wires and ruin it. Therefore, one must clean it, heat it for an hour, and then cover it with aluminum foil to separate the plata from the Pesaĥ pots.