Commentary for Avodah Zarah 151:17

The amoraim here explain how the two major forms of kashering are performed.How does one kasher a large pot, one that cannot be boiled inside a larger pot? One makes a rim of dough around its edges and then builds it up so that the water basically boils over. The rim will get hot water splashed on it. This is sufficient to kasher it, for just as the rim absorbed taste by having the forbidden substance splash on it, so it can emit taste in the same way.