The Mishna on our daf presents a case where two brothers are married to two sisters, and one of the sisters is still a minor who is married only a Rabbinic level. Should the brother who is married to the adult sister pass away, we are faced with an awkward situation: the surviving brother is married to one sister on a Rabbinic level and has a relationship of yibum (levirate marriage) to the other sister on a Torah level.
Three suggestions are raised in the Mishna:
Rabbi Eliezer suggests that we teach the girl to do mi’un (refusal), dissolving the Rabbinic marriage, which will then allow for yibum to be performed.
Rabban Gamliel rules that the girl can do mi’un if she wants. Otherwise we will wait until she matures, when her marriage will take on full significance and her older sister will then be permitted to marry anyone.
Rabbi Yehoshua says that the surviving brother cannot remain married to either one. He will have to divorce his wife (since she is the sister of his yevama, with whom he has a Torah-level relationship) and do halitza (ceremony releasing one from yibum) with the yevama (since she is the sister of his divorced wife on a Rabbinic level).
The Gemara questions how Rabbi Eliezer can recommend mi’un, given the statement of bar Kappara that mi’un is an example of something that should always be avoided. The Gemara responds that in this situation, where mi’un will facilitate a mitzva, it can be encouraged.
The Talmud Yerushalmi also teaches that a Jewish court will never encourage mi’un, and that we always find mi’un discussed as an initiative of the girl. Rabbeinu Hananel relates a tradition from the time of the Ge’onim that in order to avoid the possibility of mi’un, the practice of marrying off an underage girl was abandoned. According to the Maharshal, the Ashkenazi leadership declared a ban on anyone who does mi’un – although this was a topic of some dispute, as is clear from the Rema, Even ha-Ezer 155:10.