Doing this will allow the blood dripping from the meat to stick to the salt on the sides of the vessel, while the fat will float to the top. And when emptying the receptacle, he should tilt it gently to pour the fat into another vessel without it mixing with the blood.
אמר ליה רב אחא בריה דרב איקא לרב אשי ומי אמר שמואל הכי והאמר שמואל ככר שחתך עליה בשר אסור לאכלה שאני התם דאגב דוחקא דסכינא פליט
Rav Aḥa, son of Rav Ika, said to Rav Ashi: And did Shmuel really say this, that once the meat’s ruddiness has dissipated it is permitted to place a receptacle underneath it? But didn’t Shmuel say: It is prohibited to eat a loaf of bread upon which one cut roasted meat? Apparently he holds that meat contains blood even after it has finished roasting. Rav Ashi replied: Shmuel holds that the meat no longer emits blood after its ruddiness has dissipated. And as for his ruling with regard to a loaf of bread, it is different there, as due to the pressure of the knife the meat expels more blood.
אמר רב נחמן דגים ועופות שמלחן זה עם זה אסורין היכי דמי אי בכלי שאינו מנוקב אפי' עופות ועופות נמי אסירי אי בכלי מנוקב אפי' דגים ועופות נמי שרי
§ Rav Naḥman says: If one salted fish and birds together, the fish are prohibited for consumption due to the blood they absorb from the birds. The Gemara asks: What are the circumstances of this ruling? If they were placed in an unperforated vessel, in which the blood emitted by the birds will pool, then even if one salted a bird and another bird together they should be prohibited, as the blood expelled from one bird will be absorbed by the other. And if the case involves a perforated vessel, out of which the blood can flow, then even if fish are salted with birds the fish should be permitted.
לעולם בכלי מנוקב ודגים משום דרפו קרמייהו קדמי ופלטי ועופות קמיטי בתר דניחי דגים פליטי עופות והדר בלעי מיניה
The Gemara answers: Actually, the ruling is referring to a perforated vessel. And the fish are prohibited because their skin is soft, and therefore when they are salted they expel their blood first, whereas the skin of birds is hard. After the fish finish expelling their blood, the birds continue to expel their blood, and then the fish absorb blood from them. If one salts two birds together, they expel their blood simultaneously, and neither will absorb blood from outside while expelling its own.
רב מרי בר רחל אימלח ליה בשר שחוטה בהדי בשר טרפה אתא לקמיה דרבא
The Gemara relates: Meat of a slaughtered animal was salted for Rav Mari bar Raḥel together with prohibited meat of an animal that had a wound that would have caused it to die within twelve months [tereifa], in a perforated vessel, as meat is generally salted. He came before Rava to ask whether the kosher meat was prohibited, as perhaps it would not have absorbed flavor from the non-kosher meat while itself expelling blood.
אמר ליה (ויקרא יא, כו) הטמאים לאסור צירן ורוטבן וקיפה שלהן
Rava said to him: When the verse states, with regard to forbidden foods: “These are the unclean to you” (Leviticus 11:31), the added definite article serves to prohibit not only their flesh but also their juice emitted by salting, and their gravy, and their spices with which they are cooked. Even while the kosher meat is emitting its blood, it still absorbs the juices of the non-kosher meat, which are absorbed more easily than blood.