Before us is Mishnah 7 of chapter 14 of Maseches Yevamos, which deals with two brothers, one a deaf-mute and one of full capacity, and with the laws of yibum and chalitzah that follow from their differing status.
The case before us:
"Shnei achim, echad cheresh v'echad pike'ach" - two brothers, one of them a deaf-mute and the other of full capacity, that is, an ordinary person.
"Nesuim lishtei nochriyos pikchos" - "nochriyos" means that the women are not related to one another, and in particular that they are not sisters. "Pikchos" - both are ordinary women, fully functioning.
"Meis cheresh ba'al hapikachas":
The deaf-mute, whose marriage is only of rabbinic force, died, and his wife falls before the brother of full capacity. "Mah ya'aseh pike'ach ba'al hapikachas? O choleitz o meyabeim" - the choice is his, since he is of full capacity and there is no impediment whatsoever. There is no concern here of his wife's sister, since the women are not sisters to one another.
"Meis pike'ach ba'al hapikachas":
The brother of full capacity, whose marriage is of Torah force, died, and his widow falls for yibum before his deaf-mute brother. The deaf-mute's own marriage has no bearing on our case - he is married to a different woman, and even though his marriage is only of rabbinic force, that presents no problem. Therefore the Mishnah says: "Koneis" - he must perform yibum, and he cannot perform chalitzah. But the Mishnah adds: "V'ein motzi l'olam" - he can never release himself from her with a get.
Why can he not give a get? Because the get of a deaf-mute is valid only by rabbinic force. He can dissolve a marriage that is rabbinic, but not a marriage that is of Torah force. And here, the bond between him and his yevamah stems from a Torah-level bond, since the obligation of yibum rests upon him from the Torah, because his brother's marriage was a Torah marriage. The result is that she falls before him for yibum and he performs yibum, but he has no way to release himself from this bond by giving a get, since the get has no force against a bond that is from the Torah.
In summary: in this Mishnah we learned about two brothers, a deaf-mute and one of full capacity, married to two women of full capacity who are not related to one another. When the deaf-mute dies, his brother of full capacity performs chalitzah or yibum as he wishes. And when the brother of full capacity dies, his deaf-mute brother performs yibum and does not perform chalitzah, and he also can never release her, because his get is of rabbinic force while his bond to her is of Torah force.