1) "TOCH KEDEI DIBUR K'DIBUR DAMI"
"OPINIONS: The Gemara mentions that the principle of "Toch Kedei Dibur k'Dibur Dami" applies for all Halachos except for blaspheming (Megadef), idolatry (Avodah Zarah), marriage, and divorce.
What is the source for the principle of "Toch Kedei Dibur k'Dibur Dami," and what is the source for the exceptions? The Rishonim differ with regard to the source and with regard to the way in which "Toch Kedei Dibur" functions.
(a) The RAN writes that the principle of "Toch Kedei Dibur" and its exceptions are mid'Oraisa. The Torah assumes that a person is never totally committed to his speech or actions and he always reserves the right to retract within the small amount of time of "Toch Kedei Dibur." However, when he performs actions which are of such a severe nature (the exceptions mentioned in the Gemara), he does not begin the action until he is absolutely committed to doing it, and therefore he does not reserve in his mind the right to retract.
(b) The Ran quotes the RAMBAN in Bava Basra who says in the name of RABEINU TAM that the Halachah of "Toch Kedei Dibur" is a Takanah d'Rabanan. The Rabanan instituted this principle in order to enable a buyer to greet his teacher while in the midst of a purchase in such a way that his greeting does not constitute an interruption between the words he said just before and the words he said afterwards. TOSFOS cites this opinion in the name of Rabeinu Eliezer. Tosfos asks, however, that "Toch Kedei Dibur" cannot be a Takanah d'Rabanan because it applies even with regard to Halachos that are mid'Oraisa.
(c) The RASHBAM in Bava Basra (129b) writes that the principle of "Toch Kedei Dibur" is mid'Oraisa in all cases, including the exceptions mentioned in the Gemara. Mid'Oraisa, one may retract within Toch Kedei Dibur even in cases of Megadef, Avodah Zarah, marriage, and divorce. However, the Rabanan enacted that
"Toch Kedei Dibur" does not work in those cases. They enacted that it does not work in cases of Megadef and Avodah Zarah because of the severity of the act. They enacted that it does not work in cases of marriage and divorce in order to prevent rumors from spreading which would ruin the reputation of the children born from the union.
According to the explanations of the Ran and Rabeinu Tam, the cases which the Gemara here mentions as cases in which "Toch Kedei Dibur" applies are problematic. The Gemara says that if one tears Keri'ah for a relative before the relative dies, and then the relative dies within "Toch Kedei Dibur" of the Keri'ah, the mourner need not tear again. The reasoning of the Ran certainly does not apply; the person who tears Keri'ah does not transact any sort of deal which he might wish to retract. Rather, at the time he tore Keri'ah he was not yet obligated to tear because the relative had not yet died, and thus his Keri'ah should be ineffective. Similarly, in the case of a man who annulled a Neder under the mistaken assumption that it was his wife who made the Neder, and "Toch Kedei Dibur" he discovered that it was actually his daughter who made the Neder, the Gemara applies the principle of "Toch Kedei Dibur" even though there is no question of indecision but merely a lack of knowledge (which was rectified only after the act).
According to Rabeinu Tam, who explains that the principle of "Toch Kedei Dibur" is a Takanah d'Rabanan in case one needs to greet his teacher in the middle of a transaction, the case of Keri'ah is similarly problematic because there is no reason to apply the principle of "Toch Kedei Dibur" there.
How do the Ran and Rabeinu Tam explain these cases in which "Toch Kedei Dibur" applies?
TOSFOS in Bava Kama (73b) answers that although Rabeinu Tam's reasoning for "Toch Kedei Dibur" does not apply to Keri'ah, nevertheless the Rabanan instituted "Toch Kedei Dibur" as a special leniency in the Halachah of Keri'ah. TOSFOS in Bava Basra (129b) answers that "Toch Kedei Dibur" applies to Keri'ah because the Rabanan instituted a "Lo Plug" -- since in some situations the rule applies, they enacted that it should apply in all situations.
These answers do not answer the questions on the Ran's opinion. How does the Ran understand the application of "Toch Kedei Dibur" in the cases of Keri'ah and Hafarah?
It must be that the Ran understands that the basic principle of "Toch Kedei Dibur k'Dibur Dami" is a universal rule which states that whatever occurs within the time frame of "Toch Kedei Dibur" is non-sequential; there is no chronological order to all of the events which occur "Toch Kedei Dibur." Therefore, the Keri'ah is considered as though it happened after the death. The Ran presents his reasoning only in order to differentiate between retraction from normal acts and transactions, where the principle of "Toch Kedei Dibur" applies, and Avodah Zarah (and Megadef), Gitin, and Kidushin -- acts which are final and irrevocable once performed due to their severity, and which therefore are excluded from the rule of "Toch Kedei Dibur k'Dibur Dami." Viewing the two acts as though they happened at the same time would not allow the retraction to be considered the final status if not for the Ran's logic that the person does not fully consider the ramifications of his statement or act until after "Kedei Dibur." (Such an approach is found in the DIVREI YECHEZKEL 27:1 with regard to another question.)