(Rabah bar Yitzchak): Avraham was not commanded to do Pri'ah (to reveal the crown by peeling back the layer that remains after cutting off the foreskin) - "At that time Hash-m said to Yehoshua, make sharp knives (to circumcise Yisrael again)".
Question: Perhaps this was for those who were never circumcised - "... Those born in the Midbar had not circumcised"!
Answer: It says "Again". We must say that this time, Pri'ah was done.
Question: Why does it say "A second time"?
Answer: This equates the end of circumcision with the beginning. Just like the start of circumcision is Me'akev (one is an Arel if it was not done), also the end (Pri'ah) is Me'akev.
(Mishnah): Strands of skin that covers most of the crown invalid Milah. The man may not eat Terumah.
(Ravina): This refers to skin that covers most of the height of the crown.
Shabbos 133a (Mishnah): One may do all needs of Milah on Shabbos - cutting the foreskin, Pri'ah...
137a (Mishnah): If he circumcised but did not do Pri'ah, it is as if he did not circumcise.
Rif (Shabbos 52b), Rambam (Hilchos Milah 2:6) and Rosh (Shabbos 19:2): One may do all needs of Milah on Shabbos - cutting the foreskin, Pri'ah...
Ran (DH Masnisin): Pri'ah is peeling off the skin that covers the end of the Ever
Rif and Rosh (Shabbos 55b and 19:9): If he circumcised but did not do Pri'ah, it is as if he did not circumcise. Skin that covers the majority of the height of the crown invalidates Milah. This is the Halachah.
Rosh: The Rashbam instituted a custom for the father to bless '...v'Tzivanu Lehachniso bi'Vriso Shel Avraham Avinu' before the Milah, for all Berachos must be before the Mitzvah. R. Tam defended the custom to bless right after the Mohel blesses '...v'Tzivanu Al ha'Milah'. The Gemara supports this. The Berachah must be before the Mitzvah only when the one who does the Mitzvah blesses. Also, this Berachah praises Hash-m for Milah in general. Also, I heard that this is before the Mitzvah, for it is before Pri'ah, and Milah without Pri'ah is invalid.
Rambam (2:2): To circumcise, one cuts all the skin covering the crown until he totally reveals it. Then he removes the soft layer underneath the skin with his fingernail and pulls it back on both sides until the crown can be seen.
Rambam (2:4) If he circumcised but did not do Pri'ah, it is as if he did not circumcise.
Or Some'ach (1:9 DH): If Milah was done on Yom Tov during the day and Pri'ah was at night (even according to the opinion that permits Milah at night after day eight), there was no Heter for Chilul Yom Tov. The Mitzvah of Milah depends on when it is finished, and this was on day nine.
Question (Or Some'ach 2:1) If the only person Kosher to circumcise cannot do the entire Mitzvah, but he could finish it if someone else (Pasul to circumcise) began, should they do so, or should they wait for a Kosher person who can do the entire Mitzvah? If strands were left that invalidate Milah, one goes back to cut them even on Shabbos. This suggests that the Mitzvah is finishing Milah. If Milah was done before eight days and strands remain, they should be cut on day eight (Rema YD 264:1). Surely, this is Docheh Shabbos. However, if the Kosher person can do only Pri'ah and now that skin is covered with Orlah, perhaps the Chiyuv Pri'ah is not Chal yet!
Answer (Or Some'ach): Rav (Avodah Zarah 27a) disqualifies women for Milah. The Gemara asked from Tziporah, who circumcised her son. One answer is that Moshe finished it. This shows that we do not wait for a Kosher person to do everything. Even though Avraham was not commanded to do Pri'ah, now that we are commanded, it is part of the Mitzvah of Milah. Pischei Teshuvah (YD 264:3) says that if a slave, woman or child, and all the more so one who is Vadai Pasul, began to do Milah, if a Kosher adult can finish it he must.
Yam Shel Shlomo (Yevamos 8:3): Sefer ha'Terumos allows one Mohel to do Milah and another to do Pri'ah even on Shabbos. We do not say that the first is liable for Chilul Shabbos without the Mitzvah. Mahari Berona disagrees, for Milah without Pri'ah is no Mitzvah. One who circumcised and left strands is not Chayav Kares; he can say, 'I did half the Mitzvah' (Shabbos 133b). Milah without Pri'ah is worse; it is not even half a Mitzvah. Regarding leaving strands it says 'they are Me'akev', but if Pri'ah was not done 'it is as if he did not circumcise (at all).'
Shulchan Aruch (YD 265:1): The father blesses 'Lehachniso bi'Vriso...' after the Orlah is cut and before Pri'ah.
Question (Chasam Sofer YD 248 DH v'Hinei Pashut): The Rema (3) says that if one goes back to cut strands that are Me'akev Milah, he repeats al the Berachos. Surely the same applies to strands that Me'akev Pri'ah, for it is included in Milah and it is Me'akev. In truth, it is difficult why one should repeat 'Lehachniso bi'Vriso', since Avraham was not commanded to do Pri'ah. This implies that the Bris (covenant) is only on Milah, not on Pri'ah! However, our custom is to say the Berachah between Milah and Pri'ah, and Rishonim consider this to be before the Mitzvah. We rely on them, even though I cannot properly answer this question.
Answer (Hagahos v'Ha'aros in Tur ha'Shalem YD 265:16): The Yerushalmi (Yevamos 8:1) and Medrash Rabah (Sof Parashas Lech Lecha) expound Pri'ah from "Himol Yimol", which was said to Avraham. The Mizrachi (Bereishis 17:24, in one answer) and Pnei Moshe say that Rav, who taught that Avraham was not commanded to do Pri'ah, expounds differently. The Eshkol (Milah p. 121) says that according to R. Chananel's text (brought in Tosfos Yevamos 71b DH b'Kuntres), the Gemara concludes that Avraham was commanded about Pri'ah.
Taz (1): When the father himself circumcises, he should not say the Berachah in between. The baby is in pain, and perhaps it will be hard to do Pri'ah afterwards because the area will be covered with blood! Also, one of the Heterim to bless after Milah (the Mevarech is not doing the Mitzvah) does not apply. Rather, he should bless before the Milah.
Yam Shel Shlomo (Yevamos 8:3): Bereishis Rabah (47:8, cited in Rashi 17:25) says that Avraham cut his Orlah but did not need to do Pri'ah because the skin had already become compressed through Bi'ah over the course of many years. The Re'em says that the same applies to any adult that circumcises. He bring a proof from Keti'a bar Shalom (a Nochri who was killed for defending Yisrael. Before his death he circumcised himself, but did not do Pri'ah.) The Mishnah does not distinguish between adults and children. Also, perhaps the Medrash means that because Avraham was an adult he did not feel much pain from Pri'ah, but for children the primary pain is Pri'ah, like we see nowadays. Perhaps Keti'a bar Shalom did Pri'ah, since this is easy for an adult; the Gemara did not need to mention this. Alternatively, he did as much as he was able to do.
Chasam Sofer (YD 249 DH Emnam): The Gemara said that Milah is Docheh Shabbos because 13 covenants were made on it, and one is Chayav Kares for not circumcising. Pri'ah is also Docheh Shabbos, even though covenants were not made on it. Since the Milah itself was Docheh Shabbos, also Pri'ah is. Also, surely there is Kares for not doing Pri'ah, for it is as if one did not circumcise.
Shulchan Aruch (266:2): Even when Milah is on the eighth day, only the Milah itself, Pri'ah and sucking out the blood are Docheh Shabbos or Yom Tov.