The stream of liquid poured from a kli rishon is called iruy kli rishon. A utensil which gets used through iruy, such as a bowl into which hot soup was poured directly from a pot, can be koshered by pouring water from a kli rishon onto it. If there is reason to believe that the utensil was used to heat up food, such as a bowl that may have been placed on a hot plate on Shabbat to warm up a challah, the utensil needs hagala in a kli rishon. This is according to the ruling of the Rama regarding rov tashmisho, that even if a utensil was only used once for a higher level of cooking, it still requires that higher level of koshering. Sefardim rely on the Shulchan Aruch that we choose the method of koshering a utensil based on how it is primarily used.