We are learning Maseches Nazir, chapter 4, mishnah 5, which deals with the question of how late a husband may still annul his wife's nezirus.
The mishnah presents the following case: a woman who was a nezirah and went through the entire process of nezirus, and at the end of it brought her korbanos to the Beis HaMikdash. One of the korbanos was slaughtered and its blood was thrown, and once the blood was thrown against the mizbeach, the throwing takes effect and the korban is valid. Under these circumstances the husband can no longer annul the nezirus, because the nezirus has completed itself.
The reason for this:
The only kind of vow a husband may annul is a vow of self-affliction, a vow that causes his wife distress. And the halachah is that once the blood of one of the korbanos has been thrown, from that moment the woman is permitted to drink wine and is no longer prohibited in it. Therefore it is no longer considered a vow of self-affliction that afflicts her, and the husband can no longer annul it, since nothing of self-affliction remains in it.
The opinion of Rabbi Akiva:
Rabbi Akiva disagrees and says: "Afilu nishchetah aleha achas mikol habehemos - eino meifer" - even if only one of the animals has been slaughtered on her behalf, he cannot annul. It is enough that one of the animals has been slaughtered, and even though the blood has not yet been thrown, the husband can no longer annul. The reason is the concern for the loss of kodashim: if he were to annul at that point, the korbanos that have already been slaughtered would become invalid, and we do not want to see them go to waste.
"Bameh devarim amurim" - in what case was this said:
When exactly is it that the husband can no longer annul? In the case of the shaving of purity, when these korbanos are brought as part of the process of the shaving of purity, in which the woman shaves her head at the end of her days of nezirus and brings korbanos on account of being tahor. This is in contrast to a situation of tumah, in which a person is required to undergo a process of shaving the head and bringing korbanos in the middle of the nezirus, on account of having become tamei.
And therefore the mishnah says: "Aval betiglachas hatumah - yafer" - but in the case of the shaving of tumah, he may annul. If we are dealing with the shaving of the head and the bringing of korbanos because she became tamei, and for that reason she must begin her nezirus all over again, the husband may still annul. Since she must continue her nezirus, he can say: "Ee efshi be'ishah menuvvelet" - I do not want a wife who is a nezirah. And this is like any case in which a husband may annul a vow of nezirus, because the woman is causing herself distress by not allowing herself to drink wine.
The opinion of Rabbi:
Rabbi says that even when the woman shaves and brings her korbanos of purity, and even after the blood has been thrown, as long as she has not yet shaved her hair - "Yafer" - he may annul, and the husband may still annul, for a different reason. Even though she is already permitted to drink wine, the husband still has a legitimate reason to want the nezirus not to take effect, for he can say: I do not want a wife whose head is shaved. And if he annuls the vow, she will not need to shave her head. So even though the blood has already been thrown and the nezirus is, as it were, complete, this aspect of the nezirus has not yet been completed, and the husband seeks to stop the matter where it stands.
The Tanna Kamma, on the other hand, is not concerned about this, as the Gemara explains: he says that the woman can wear a wig, and there is nothing here that would bother the husband to that extent.
In summary: there are three opinions as to how late the husband may annul:
Tanna Kamma: until the throwing of the blood, since once the blood has been thrown the woman is permitted in wine and there is no longer any self-affliction.
Rabbi Akiva: until the slaughtering of one of the animals, because of the concern for the loss of kodashim.
Rabbi: even after the throwing of the blood, as long as she has not yet shaved, because he can say that he does not want a shaved wife - and the Tanna Kamma is not concerned about this, since she can wear a wig.
And all of this applies to the shaving of purity; but in the case of the shaving of tumah, where the nezirus continues, the husband may always annul.