If the blood of a valid korban (sacrifice) became mixed with the blood of an invalid korban—for example, if you had a korban slaughtered with improper intentions, took its blood, and mixed it with the blood of a regular valid korban—you are not allowed to use it for zerikah (the throwing of the blood). Yishafech la'ama; rather, it should be poured into the pipe of water that flows all the way to the courtyard of the Beit HaMikdash.
Be-dam ha-tamtzit (draining blood), yishafech la'ama: same thing. If you have blood from a valid korban that became mixed with draining blood, what would happen is that after the shechita (slaughter), there would be the dam ha-nefesh, the blood of the animal that would actually squirt out. That blood is used for zerikah. But the blood here, which we call dam ha-tamtzit, or draining blood, is just the little bit of blood that drained out of the neck; it didn't squirt out. That type of blood is not valid for the zerikah, the throwing of the blood on the Mizbeach (altar). So again, the case is valid korban blood mixed with draining blood, and the draining blood is unfit to be poured onto the Mizbeach. We say yishafech la'ama: it should be poured into that water that drains into the courtyard of the Beit HaMikdash. That's where it should be poured. It's not fit for zerikah, not on the Mizbeach, and that's what we do.
In both these cases, the Hachamim (rabbis) made a decree. They said, “We're dealing with cases that are unlike the previous cases.” In the previous cases, everybody knows that water is not kosher to be used on the Mizbeach, and everybody knows that chulin, blood from a regular animal, is not kosher to be used on the Mizbeach, so therefore we could say that there's bitul, that they become nullified in some way. But in these cases, these two that we brought—which are the blood of an invalid korban or the draining blood that came out of the animal's neck—people will mistakenly think that it's okay, that those types of blood are allowed to be used on the Mizbeach. They may make that mistake. So what did the rabbis do? They went strict and said that when it comes to the blood of these animals, if it got mixed in in any capacity with regular blood, we have to make a decree that they are not allowed to be used on the Mizbeach, because if we allow them to be used on the Mizbeach, people are going to think that it's permissible to pour this type of blood on the Mizbeach. So it is fully outlawed by the Tana Kama, by the Hachamim here.
Rabbi Eliezer says that in the second case, which is the mixture of valid blood and the draining blood that came out of the animal's neck, the blood is fit for zerikah on the Mizbeach. According to Rabbi Eliezer, he understands the decree of the Hachamim to mean that they are nervous that a mixture of invalid blood and blood is going to be used on the Mizbeach. But in that type of situation, the issue would only exist if most of the mixture is blood from an invalid korban. That's when people are going to conclude, “Ah, the majority is from the blood of an invalid korban, and that could still be used on the Mizbeach, so maybe an invalid korban in some capacity could be used.” That's when you have a real chashash (concern). But according to Rabbi Eliezer, if you're talking about draining blood, draining blood is a tiny bit that comes out afterwards. So it's always going to be a tiny amount of blood. Therefore, the decree of the rabbis did not extend to a mixture of regular blood and draining blood, because the draining blood will never be the majority. Therefore, he says that in that type of situation, you could throw that type of blood onto the Mizbeach.
Now we'll go back to the Tana Kama. Im lo nimlach v'natan kasher (valid). The first Tana Kama would agree that, with a korban, he did what he wanted to do. He never asked the rabbi what to do in this type of situation. Then the Tana Kama says that if he went ahead and just threw the blood over there, kasher, the zerikah is valid after the fact. Since the mixture technically on a biblical level is okay, and it's only prohibited by the rabbis, after the fact when it was done, we're not going to decree and we're going to say that the zerikah is in fact valid.