Yevamos, Chapter 13, Mishnah 3. According to the understanding of this Mishnah presented here, which is based on one of the opinions in the Gemara, the Mishnah distinguishes between two ways of undoing rabbinic kiddushin that were made for a minor girl by her mother or her brother:
Mi'un - the girl herself refuses and thereby annuls the kiddushin, as described earlier.
Get - the husband gives her a get and thereby ends the marriage.
The words of the Mishnah:
"Rabbi Elazar ben Yaakov omer: kol ikuv shehu min ha'ish - k'ilu hi ishto; v'chol ikuv she'eino min ha'ish - k'ilu einah ishto" - Rabbi Elazar ben Yaakov says: whenever the holding back comes from the man, it is as though she is his wife; and whenever the holding back does not come from the man, it is as though she is not his wife.
"Kol ikuv shehu min ha'ish":
The holding back, meaning the reason the couple either stays together or separates, comes from the man. This is another way of saying that he ended the marriage by giving a get. In this case we view the situation as though she truly was his wife: the marriage was a valid marriage, and once it was dissolved with a get, both of them remain forbidden to the relatives of the other side, since they were once husband and wife.
"Kol ikuv she'eino min ha'ish":
The holding back is not based on him but on her, which is another way of saying that she performed mi'un and annulled the kiddushin. In this case we view the situation as though they were never married at all. Therefore, once she has refused, he is permitted to marry her relatives and she is permitted to marry his relatives, since the marriage is not regarded as having been valid at any stage.
In the next Mishnah this distinction will be applied in practical terms.