QUESTION: The Gemara derives from the verse, "v'Yibmah" (Devarim 25:5), that one may perform Yibum against the will of the woman. RASHI (DH Ba'al Korchah) explains that this means that even if the brother marries her against her will, the marriage takes effect "with regard to Yerushah (inheriting her possessions upon her death), Tum'ah (the allowance to bury her and become Tamei if he is a Kohen), and with regard to all matters, she is his wife."
Why does Rashi specify that the verse means that the marriage done against her will takes effect for all matters of Kinyanim (acquisitions)? Rashi should explain instead that it takes effect for all of the laws of Ishus, marriage. The Gemara says merely that the Yavam fulfills the Mitzvah of Yibum even though he did the act without her consent, and now he must give her a Get if she wants to remarry; Chalitzah no longer suffices, since the act of Yibum was performed. Why does Rashi describe this Halachah in terms of Kinyanim, and not in terms of fulfilling the Mitzvah of Yibum? (HAGAHOS MAHARSHAM)
ANSWER: Rashi is bothered by a question. Why does the Gemara need the verse of "v'Yibmah" to teach that the Mitzvah of Yibum is fulfilled against the will of the woman? No verse is necessary to teach this law, because the verse that introduces the Mitzvah of Chalitzah states explicitly, "Im Lo Yachpotz ha'Ish..." -- "If the man does not desire to marry his Yevamah..." (Devarim 25:7). The verse clearly makes the fulfillment of the Mitzvah dependant upon the man's will; if he does not want to perform Yibum, then they perform Chalitzah, and if he wants to perform Yibum, then they perform Yibum regardless of what the woman wants. Why, then, is the verse of "v'Yibmah" necessary to teach that he fulfills the Mitzvah against her will?
Rashi answers that one might have thought that although the Mitzvah of Yibum may be performed against her will, such an act of involuntary Yibum affects only the laws of marriage but has no bearing on matters of Kinyanim. Rashi explains that the Gemara finds a source ("v'Yibmah") which proves that even with regard to Kinyanim, Yibum without the woman's consent is effective.