1) WHY A MAN CANNOT DO YIBUM WHILE ASLEEP
QUESTION: The Gemara says that if a Yavam has relations with his Yevamah while he is asleep, the Yibum does not take effect. RASHI (DH Yashen Lo Kanah) explains that "a sleeping person is not a Bar Da'as (he has no mental competence) and his Kinyan is not a valid Kinyan."
Why is this a valid reason for why a sleeping Yavam is not Koneh the Yevamah? There are other situations in which the Yavam performs an act of Bi'ah with the Yevamah but does not have intention to be Koneh her, and nevertheless the Yibum is valid. For example, when a man performs an act of Bi'ah inadvertently (b'Shogeg) or against his will (b'Ones), he does not have in mind to be Koneh the Yevamah and nevertheless he fulfills the Mitzvah of Yibum.
Also, the law is that when a Katan (minor) -- who does not have Da'as to make a Kinyan -- performs an act of Yibum, the Yibum takes effect (mid'Oraisa, according to Rashi and other Rishonim; see Insights to Yevamos 39:2). (Even if the Katan is not Koneh her mid'Oraisa, it is only because the Torah specifically excludes him from Yibum and not because he lacks Da'as.) (TOSFOS DH Yashen, RASHBA)
(a) The RITVA answers that Bi'as Shogeg and Bi'as Ones inherently differ from the Bi'ah of a sleeping person. In the cases of Bi'as Shogeg and Bi'as Ones, the Yavam is capable of having conscious intent at the moment of the act to be Koneh the Yevamah, but he chooses not to have in mind to be Koneh her. In contrast, a man who is sleeping is not capable of having any conscious intent altogether. (Rashi emphasizes this point when he adds the words, "and his Kinyan is not a valid Kinyan" -- that is, he is not capable of making a Kinyan while he is asleep.)
Why, though, is a sleeping man different from a Katan? A Katan is also not capable of making a Kinyan, and yet he nevertheless is able to be Koneh a Yevamah. The Acharonim (see YA'AVETZ) answer that although a Katan might not be able to make an ordinary Kinyan, he certainly does have the ability of conscious thought and intent which can have Halachic ramifications (see Chulin 13b). The ARUCH LA'NER adds that there are times when even a Katan can make a Kinyan (see TOSFOS to Sanhedrin 69b). Accordingly, a Katan is not comparable to a sleeping person who is entirely incapable of making any type of Kinyan.
(b) TOSFOS (DH Yashen) and other Rishonim do not accept Rashi's reasoning. They explain that a sleeping person cannot be Koneh his Yevamah for a different reason. The Gemara later says that although a person does not need conscious intent, Da'as, to be Koneh a Yevamah, he does need intent to do an act of Bi'ah. A person who is sleeping has no intent whatsoever to do an act of Bi'ah. A Katan, however, can have intent to do Bi'ah even though he does not have Da'as, the ability to make a deliberate decision to effect a Kinyan.
(Rashi perhaps disagrees with this contention and maintains that even one who is asleep is capable of having intent to do Bi'ah; see Nidah 43a.)
2) THE SOURCE THAT "BI'AH SHE'LO K'DARKAH" IS AN EFFECTIVE FORM OF YIBUM
QUESTION: The Gemara derives from several phrases in the verse (Devarim 25:5) that a person is Koneh his Yevamah with various types of Bi'ah -- such as Shogeg or Mezid, Ones or Ratzon, k'Darkah or she'Lo k'Darkah. The phrase, "Yevamah Yavo Aleha" teaches that the act of Yibum takes effect whether it is done b'Shogeg or b'Mezid, b'Ones or b'Ratzon. The phrase "u'Lekachah" teaches that he is Koneh the Yevamah even with an act of Lo k'Darkah.
The Gemara questions this teaching from a Beraisa which says that the phrase "Yevamah Yavo Aleha" teaches that Bi'ah k'Darkah is Koneh. How, then, can the same phrase teach that the act of Yibum is Koneh whether it is done b'Shogeg or b'Mezid?
What is the Gemara's question? If the verse teaches that the Yavam is Koneh the Yevamah with Bi'ah whether it is done b'Shogeg or b'Mezid, then obviously the verse includes Bi'ah k'Darkah. Why would one have assumed that the verse refers to any type of Bi'ah other than Bi'ah k'Darkah?
ANSWER: The RASHBA explains that the Gemara's question is that the laws of Bi'ah b'Shogeg and b'Mezid, and k'Darkah and she'Lo k'Darkah, cannot be derived unless there is a verse which teaches that the Yavam is Koneh the Yevamah with a normal Bi'ah k'Darkah. Only when there is a verse which teaches how Yibum is to be done in the first place (i.e. with Bi'ah) can another verse teach that this Bi'ah is effective when done in different ways, such as b'Shogeg and she'Lo k'Darkah.
The Gemara's question is that the phrase "Yevamah Yavo Aleha" is the first verse which teaches how the Mitzvah of Yibum is to be done in the first place (with Bi'ah). How, then, can the same verse be used to teach that Bi'ah b'Shogeg also works? Bi'ah b'Shogeg must be derived from a different verse (such as "u'Lekachah"), in which case there will be no additional source to teach that the Yavam is Koneh the Yevamah with Bi'ah she'Lo k'Darkah. That is the Gemara's question.
The Gemara answers that there is a third verse -- "l'Hakim l'Achiv Shem." That verse teaches that Yibum is accomplished through Bi'ah (k'Darkah). The other two verses teach that the additional forms of Bi'ah (b'Shogeg and she'Lo k'Darkah) also accomplish Yibum.