Yevamos, Chapter Five, Mishnah 2. The previous Mishnah presented the dispute between Rabban Gamliel and the Sages as to whether a get after a get, or a ma'amar after a ma'amar, has any legal effect. Our Mishnah does open with the word "keitzad" ("how so"), and one might have thought that it comes to spell out an example of that dispute, but the Gemara explains that this is not the case.
Our Mishnah deals with the simple case of one yavam and one yevamah, and it comes to teach how the relationships between four acts operate: a get, a ma'amar, marital relations, and chalitzah.
The three cases in the Mishnah:
"Asah ma'amar biyvimto v'nasan get - tzerichah heimenu chalitzah" - the yavam performed a ma'amar, which is a rabbinic betrothal, and afterward gave a get. As mentioned in the previous Mishnah, every ma'amar requires a get in order to remove that rabbinic betrothal. Even so, she still requires chalitzah, because the get does not remove the zikah. The zikah, that is, the bond created through the yavam, remains fully in place, and therefore chalitzah is essential.
"Asah ma'amar vachalitzah - tzerichah heimenu get" - the chalitzah took care of the zikah, the levirate bond; but since he performed a ma'amar with her and thereby created that rabbinic betrothal, a get is needed for the ma'amar. Even though chalitzah was performed, the get is still required.
"Asah ma'amar u'va'al - harei zeh mitzvah" - when he first performed a ma'amar and afterward had relations with her, this is the mitzvah carried out properly. The Sages instituted that a yavam should precede relations with a ma'amar, and doing things in this order is the proper way to fulfill the mitzvah of yibum.
In summary: this Mishnah is not dealing with the dispute between Rabban Gamliel and the Sages, but rather with the relationship between the various acts in the case of one yavam and one yevamah: a ma'amar followed by a get requires chalitzah, since the get does not remove the zikah; a ma'amar followed by chalitzah requires a get for the rabbinic betrothal; while a ma'amar followed by marital relations is the mitzvah carried out properly.
In the next Mishnah we will return to the dispute between Rabban Gamliel and the Sages, as to whether there is a ma'amar after a get, or a get after a ma'amar, and the like.