What does ma-amar accomplish?

 When you go to a Jewish wedding, you notice that the Rabbi prepares 2 cups of wine because they signify two different ceremonies. The first ceremony is erusin or kidushin, betrothal. The second ceremony is nissuin marriage. Originally these two ceremonies were separated by a whole year. Today they had collapsed under one huppah. 

When a couple participate in the ceremony of kidushin, they're bound only to each other. A couple does not live together and do not have intimate relations nor is the groom/husband responsible for her until nessuin. Some of this is actually spelled out in the kidushin blessing recited under the huppah. "You are blessed, Lord our God, the sovereign of the world, Who has sanctified us by Your commandments and commanded us concerning [forbidden] intimate relationships and forbidden to us those women who are [only] betrothed with erusin, but permitted to us those women who are married to us by chuppah and kiddushin. You are blessed, Adonai, that sanctifies the people Israel by chuppah and kiddushin." 

Nevertheless, the bond is so strong between the couple that the laws of adultery will apply and if the man no longer wishes to participate in the nissuin ceremony, he has to give her a Jewish bill of divorce, a get. 

Yibum is analogous to nissuin. All the yavan has to do is move in with yevamah. The rabbis created a rabbinic institution ma-amar (מַאֲמָר) that will parallel the ceremony of kidushin. Today's daf TB Yevamot 29 analyzes what does ma-amar accomplish according to Beit Shammai. The Mishnah sets up a case study.

"In the case of three brothers, two of whom were married to two sisters, and one who was single, the following occurred: The husband of one of the sisters died childless, leaving behind his wife, and the single brother performed levirate betrothal [ma’amar] to this wife. The single brother performed an act of betrothal to the yevama but did not yet consummate the marriage by engaging in sexual intercourse. Afterward, the second brother died, and therefore the second brother’s wife, the sister of the betrothed, happened before the single brother for levirate marriage as well. Beit Shammai say: His wife remains with him. The woman he betrothed is considered like his wife, and he is not required to divorce her. And this other woman leaves the yavam and is exempt from levirate marriage as the sister of a wife." (Sefaria.org translation)

The Gemara comes the conclusion that ma-amar according to Beit Shammai doesn't create as strong of an between the man and the woman as does kidushin. It doesn't create a marital bond at all. Nevertheless, the bond is strong enough to block another yevama as described in our Mishna.

"§ With regard to the main issue of levirate betrothal, it would seem from this mishna that Beit Shammai hold that levirate betrothal has the legal force of levirate marriage. However, Rabbi Elazar said: Do not say that levirate betrothal acquires the woman as a full-fledged acquisition according to the opinion of Beit Shammai, such that the first woman becomes his wife to the extent that if he wants to divorce her a bill of divorce would suffice without an act of ḥalitza. Rather, for Beit Shammai, levirate betrothal acquires the woman only insofar as it precludes a rival wife from entering levirate marriage. The legal force of levirate betrothal is limited to preventing the rival wife of the sister from performing levirate marriage; she is exempt from both levirate marriage and ḥalitza." (Sefaria.org translation)