QUESTION: The Gemara discusses a case in which Reuven's wife is a Ketanah (minor) and an Ervah to Reuven's brother Shimon (for example, she is Shimon's daughter). When Reuven dies, the Tzarah must do Chalitzah. Although the Ervah normally exempts the Tzarah completely when they fall to Yibum together, in this case the Tzarah is not exempted completely because the Ketanah has the ability to uproot her relationship retroactively through "Mi'un" so that the Tzarah will no longer be the Tzarah of an Ervah.
The Gemara explains that even if the Ketanah uproots her marriage through Mi'un, the Tzarah may not do Yibum because from the time of the husband's death, the Tzarah appeared to be the Tzarah of Shimon's daughter (since his daughter originally fell to Yibum). In order to prevent people from mistakenly permitting the Tzarah of one's adult daughter who falls to Yibum, the Rabanan disallowed Yibum for the Tzarah of one's minor daughter who performed Mi'un.
RASHI (DH Hachi Nami; 12a, DH mi'She'as Nefilah and DH Mishum Tzaras Bito) explains that when the Gemara says that the girl can uproot her marriage through Mi'un, it refers to an act of Mi'un with her father to whom she fell to Yibum.
How can the girl do Mi'un with her father? She does not fall to him for Yibum at all since she is an Ervah to him. She has no Zikah to her father which would need to be removed through Mi'un!
Moreover, even if the Mi'un which the girl does is not with her father (as Rashi explains) but is with her original husband (who died), how can Mi'un be done to a marriage which no longer exists? Once her first husband dies, nothing is left of the marriage. How, then, can Mi'un be done with the first husband?
(a) The RITVA (12a) explains that Rashi is not saying that the girl does Mi'un to break the relationship that she now has with her father as a result of Zikah, because there is no Zikah to her father. Rather, Rashi means that she declares that she does not want the original Kidushin to take effect because it will cause her to fall to her father for Yibum. (See RASHI DH va'Tema'en.)
The Ritva apparently understands that Mi'un may be performed after the husband has died (and, according to the Ritva, this is the intent of Rashi).
(b) The Ritva cites other Rishonim who maintain that Mi'un works to uproot only a marriage which still exists. Mi'un cannot uproot a marriage which no longer exists (such as after the husband died). The case in which the Gemara suggests that the daughter of the Yavam does Mi'un after the death of her husband is when there are other brothers besides her father (i.e. her uncles), and thus there is a binding Zikah to the other brothers. Since there is a Zikah to the other brothers, she may do Mi'un with one of them to remove the Zikah.
(c) Rashi's words, however, do not reflect either of these explanations. Perhaps Rashi means as follows:
In an ordinary case of an Ervah who falls to Yibum, the mechanism by which the Ervah removes the Zikah from all of the Tzaros is by falling to the Yavam and then uprooting the Zikah entirely, even for the Tzaros. In contrast, when a woman who is exempt from Yibum does not fall to Yibum at all in the first place (such as an Aylonis; see Insights to Yevamos 12:1), she does not exempt the Tzaros at all. In the case of the Gemara here, the Ervah falls to Yibum and then uproots the Zikah.
When Rashi writes that the girl performs Mi'un with her father, he means that she performs Mi'un with her first husband to avoid the initial stage of falling to her father for Yibum in the first place so that she will not uproot the Zikah entirely. The Tzarah is then left with an obligation to do Yibum or Chalitzah.
How, though, can the girl perform Mi'un to end a marriage that is already over? The answer is that as long as there are some effects left over from the fact that she fell to her father for Yibum when her husband died, she still may perform Mi'un to retroactively prevent herself from falling to him. In this case, her initial falling to Yibum effected a removal of the Zikah and an exemption for her Tzarah from Yibum and Chalitzah. Since that effect remains, she still may perform Mi'un to prevent herself from falling to him and thereby obligate her Tzarah to do Yibum.
A similar concept exists in the laws of Nedarim. One may not rescind a Neder once it is no longer pertinent (for example, one vowed not to eat a certain item, he then transgressed the Neder and ate the item, and now he wants to rescind his Neder through Hataras Nedarim). If, however, witnesses warned him before he transgressed the Neder, and now he is Chayav Malkus for transgressing the Neder, he may annul the Neder to prevent the effects of the Neder (i.e. Malkus) from taking place.