1) BOWING DOWN ON A STONE FLOOR
OPINIONS: Rav Chiya bar Avin said that he saw Rava and Abaye turn to their sides when they bowed.
Why were they particular not to fully prostrate themselves?
(a) RASHI explains that they were following Rebbi Elazar's ruling (22b) that an "Adam Chashuv," a person of importance, should not bow down.
(b) TOSFOS (22b, DH v'Iy Ba'is Eima) and the ROSH explain in the name of RAV HAI GA'ON that they leaned to their sides because of the prohibition against bowing down on a stone floor.
However, if they were concerned with the prohibition against bowing on a stone floor, they could have bowed down without spreading out their arms and legs ("Pishut Yadayim v'Raglayim") -- a manner of bowing which is permitted on a stone floor, as the Gemara earlier (22b) says. Why did they lean to their sides?
The answer is that although the Torah forbids only "Pishut Yadayim v'Raglayim" on a stone floor, nevertheless they maintained that the Rabanan prohibited bowing down on a stone floor even without "Pishut Yadayim v'Raglayim."
HALACHAH: The REMA (OC 131:8) rules that bowing down is permitted only when two conditions are fulfilled: one bows without "Pishut Yadayim v'Raglayim" on a floor that is not made of stone. When only one condition is fulfilled -- either one bows with "Pishut Yadayim v'Raglayim" on a floor that is not made of stone, or he bows on a stone floor without "Pishut Yadayim v'Raglayim" -- the act of bowing is prohibited mid'Rabanan. (The MISHNAH BERURAH (131:41) points out that bowing without "Pishut Yadayim v'Raglayim" refers to bringing one's face to the floor without spreading his arms and legs, and not just to when he merely prostrates himself upon his knees). This is the source for the practice today to place a covering on the floor when one bows during the Chazan's repetition of the Yom Kippur Shemoneh Esreh, even though one does not bow with "Pishut Yadayim v'Raglayim."
One is permitted to bow while he leans to the side, even on a stone floor. However, one may not bow down on a stone floor with "Pishut Yadayim v'Raglayim" even if he leans to the side.

23b----------------------------------------23b

2) "PORSIN AL SHEMA"
OPINIONS: The Mishnah states that the act of "Porsin Al Shema" may not be done with less than ten men.
What is the act of "Porsin Al Shema"? What does the word "Pores" mean, and how is this act performed?
(a) RASHI explains that the word "Pores" comes from the word "Peras," or "half." "Pores Al Shema" means that only half of the blessings of Keri'as Shema are recited. According to Rashi (as the RAN explains), a person who prays by himself without a Minyan may not recite any Davar sheb'Kedushah, and thus he does not say Kadish, Barchu, or the Kedushah in the first blessing of Keri'as Shema (the blessing of "Yotzer Or"). If ten individuals who prayed alone then come together in the synagogue, they may appoint a Chazan to say Kadish, Barchu, and the first blessing of Keri'as Shema and its Kedushah (they recite the blessing as a "Nedavah," a freewill offering, as they already recited the blessing (without Kedushah) when they prayed by themselves earlier).
The Rishonim disagree about whether the act of "Porsin Al Shema" requires ten men who all need to hear those Devarim sheb'Kedushah because they prayed by themselves, or whether it suffices if only some of them need to hear the Devarim sheb'Kedushah (while the others already heard them):
1. Rashi writes that "a Minyan of people who come to the synagogue" are "Porsin Al Shema." He apparently maintains that "Porsin Al Shema" requires that there be ten men who all prayed by themselves and need to hear the Devarim sheb'Kedushah. The ME'IRI also records such an opinion.
2. TOSFOS cites Maseches Sofrim and says that it suffices if only six or seven of the ten men have not yet heard Kadish, Barchu, and Kedushah.
3. RABEINU TAM (cited by Tosfos) maintains that it suffices to be "Pores Al Shema" if only five of the ten men need to hear the Devarim sheb'Kedushah.
4. The TALMIDEI RASHI (cited by Tosfos) rule that a person may be "Pores Al Shema" even if he is the only one of the ten who did not yet hear the Devarim sheb'Kedushah.
(b) Other Rishonim explain that the act of "Porsin Al Shema" refers to the recitation of both blessings of Keri'as Shema and not just the first one, and it is done not by a group of ten men who prayed individually but by people who do not know the blessings by heart and do not have Sidurim, who want to fulfill their obligation by hearing one person recite the blessings. Since these blessings have the status of a Davar sheb'Kedushah, the Rabanan decreed that their recitation needs ten men; it does not suffice for one person to recite them for one other person, as may be done with other blessings. In order to fulfill the obligation to recite these blessings by listening to one person's recitation, there must be a group of ten men present. (The Rishonim compare this requirement to the law of Zimun for Birkas ha'Mazon, whereby one person may recite Birkas ha'Mazon for others only when there are at least three people present.)
What is the meaning of the word "Porsin" according to this explanation?
1. The RAMBAM (in Perush ha'Mishnayos) defines "Pores" as to "spread out" or to "prepare" (such as in the phrase, "Pores Mapah," Pesachim 100a). This also seems to be the intent of RASHI in Sotah (30b, DH k'Sofer).
2. The GE'ONIM (cited by the Ran) explain that "Pores" means to "begin": one may not begin to say Shema and its blessings with less than ten men. (This implies that if one already began with ten men, he may continue even if some leave.)
3. The ME'IRI in the name of the MICHTAM explains that "Pores" means to "bless" (to recite the blessings of Keri'as Shema).
According to the Ge'onim, the reason for why ten men are required for "Porsin Al Shema" is not the same as the reason for why ten men are required for the other acts mentioned in the Mishnah. Those other acts are genuine Devarim sheb'Kedushah, and a Davar sheb'Kedushah may be recited with only ten men in the first place. In contrast, the blessings of Keri'as Shema may be recited by an individual (who knows them by heart). Only when one person wants to recite the blessings on behalf of others who do not know them by heart are ten men required, because the blessings are "like" a Davar sheb'Kedushah.
The Gemara in Sotah (30b, the only other place where "Pores Al Shema" is mentioned) seems to contradict the view of these Rishonim. The Gemara there implies that everyone recites the words together with the Chazan (and not that they listen to him as he recites them). However, Tosfos there cites a Tosefta which indeed describes "Pores Al Shema" as an act in which the Chazan recites the words for the others who listen and fulfill their obligation with his recitation, and they do not repeat the words that he says.

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