Initial Ruling: Initially it seems that the Shochet must draw the knife a neck-length past the neck, which is the reason that this Shi'ur (of the knife) is necessary.

Amended Ruling: It appears however, that this is not necessary, and that the Shi'ur (of the knife) is only necessary in order to Shecht liberally ...

Conclusion: Sometimes in fact, it happens that the knife is so sharp that one Shechts the majority of the Simanim before one has even drawn it along the entire width of the neck.

Implied Question: The reason that the Gemara does not ask directly that we have already learned this in our Mishnah is ...

Answer #1: ... because, as Rashi explains, the Mishnah does not explicitly state 'Hipilah Hu. Kesheirah', and it is Rava who says it.

Alternative Answer: It seems however, that even if the Mishnah had explicitly said it, we would not have been able to ask ...

Explanation (Part 1): ... because the Gemara does not ask why Rava needs to extrapolate 'Ha Hipilah Hu, Kesheirah', seeing as the Mishnah has already taught us that, by virtue of the fact that it declares Kasher the Shechitah of a Chashu.

Explanation (Part 2): ... which in turn, is because, if Rava had not established that Mishnah like Rebbi Nasan, there would have been nothing to ask, due to the possibility that even the Rabbanan will agree by a Chashu, who at least have the intention of cutting flesh ...

Explanation (Part 3): ... and the Gemara only asks on Rava from there, because he establishes the author as Rebbi Nasan, from which we see that the Shechitah is Kasher even there where he did not intend to cut the flesh, just like the case (of Rebbi Nasan) where one threw a knife with the intention of sticking it into the wall ...

Explanation (Part 4): ... thereby prompting the Kashya why Rava then needs to extrapolate 'Ha Hipilah Hu, Kesheirah'?

Conclusion: And the Gemara answers that we would otherwise have thought that the case of the knife in the wall is better, since he at least had the intention to make a cut (in the wall), whereas that of Hipilah, he did not.

Observation: It transpires that this Tzerichusa is not parallel to the other one ...

Explanation (Part 1): ... since the Gemara is not coming to explain Rava, why he needs to establish the case of Chashu like Rebbi Nasan, because if it was, what does it mean when it says 'But there, where it does not come from the force of a ben Da'as, we would have thought that it is Pasul, seeing as the Mishnah specifically rules that it is Kasher?

Explanation (Part 2): The Tzerichusa must therefore pertain to the Mishnah itself, why it needs to issue the ruling at all.

Explanation (Part 3): Rava however, needs to establish the Mishnah like Rebbi Nasan, since we would otherwise have thought that even the Rabbanan will concede that it is Kasher, seeing as the Chashu had the intention of at least cutting the flesh.

Explanation (Part 4): Therefore it needs to inform us that they nevertheless declare it Pasul ...

Implied Question: ... and when Rava says later 'Granted that he does not require Kavanah for Shechitah, he does however require Kavanah to cut' ...

Answer: ... he means 'to cut specifically the Simanim'.

Implied Question: She is forbidden to eat Ma'aser (Sheini), too, as we will learn shortly ...

Answer: ... and the reason that he mentions Terumah is because it applies nowadays.

Implied Question (Part 1): Why did the Gemara not ask the other way round 'If you forbid an Isur Misah, then an Isur Kareis should certainly be forbidden'? ...

Implied Question (Part 2): ... since it is obvious that we need Kavanah for Terumah?

Source (Part 1): ... as we learned in 'Ein Dorshin (Chagigah 18b) 'Taval le'Chulin, Asur be'Ma'aser!'

Source (Part 2): ... seeing as even regarding Chulin Rava wants to say (initially) 'Taval ve'Lo Huchzak, Ke'ilu Lo Taval'.

Answer #1: The reason that he asked the way that he did is because he first mentioned 'Tehorah le'Veisah'.

Answer #2: 'Alternatively, Ne'ensah is better than Taval le'Chulin, because, if she had known, she would have had Kavanah for Ma'aser as well. And since he said 'Tehorah le'Veisah', we see that it is considered Kavanah, in which case, she ought to be permitted to eat Terumah as well.

Explanation #1 (Part 1): It would be fine if the Tana was not talking about 'Yoshev u'Metzapeh', only S'tam, as he states, since then it would seem, he is coming to teach us that Chulin does not require Kavanah.

Explanation #1 (Part 2): But if you want to force the issue and to explain that he is speaking about 'Yoshev u'Metzapeh', then what is he coming to teach us?

Explanation #1 (Part 3): ... since he cannot be coming to teach us that one needs Kavanah, seeing as the Mishnah does not specifically imply that it is speaking by 'Yoshev u'Metzapeh'; In fact 've'Nafal' suggests that it is speaking without Yoshev u'Metzapeh ...

Explanation #2: ... whereas according to the Maharich, the Gemara asks 'Mai le'Meimra' - because, since he says 'Tehorin', which implies that he is coming to teach us a Kula, the Tana cannot be coming to teach us that one needs Kavanah (which is a Chumra).

Explanation #1: Rashi explains that one cannot Tovel in a Chard'lis (rain-water rushing down a slope) because of the Mishnah in Taharos (8:9), which forbids Toveling in a waterfall or on a slope', because, he says, there is not forty Sa'ah in one spot, even though there is a lot more that forty Sa'ah above and below it.

Refutation: Rashi need not have said that however, because, irrespective of how many Sa'ah there are on the slope, it is forbidden to Tovel on a slope ...

Explanation #2: ... since a Mikvah is not Metaher via flowing water, as we learned in a Beraisa in Toras Kohanim - "Ach Ma'yan u'Bor (only a fountain and a pit)" ... 'Perhaps just as a fountain is Metaher with flowing water, so too is a Mikvah?' Therefore the Torah writes "Ach Ma'yan u'Bor" - a fountain is Metaher via flowing water, but a Mikvah only via a gathering of water.

Observation: Rashi himself cites this Toras Kohanim in 'Bamah Ishah' (Shabbos 65b DH 've'Savar').

Question: If we already know from one Mishnah that one is permitted to Tovel 'be'Roshin', why do we need a second Mishnah to teach us the same thing?

Answer (Part 1): Perhaps the Mishnah (of 'Matbilin be'Roshin') is speaking mi'd'Oraysa, similar to 'Ein Matbilin be'Kifin', which is also mi'd'Oraysa ...

Answer (Part 2): Whereas the Mishnah of 'Gal she'Nislach' is coming to teach us that the Chachamim did not forbid it mi'de'Rabbanan either.

Clarification: That is only after he removed them from the water, because as long as they are joined to the water, they cannot become Muchshar ...

Source: ... as we learned in the Mishnah in Maseches Machshirin (4:6) 'If a Nidah is washing a radish in a cave, it is Tahor; the moment she removes it from the water, even partially, it becomes Tamei' - by which the Tana means 'Muchshar ...

Implied Question: .. and when the Gemara in the first Perek of Pesachim (16a) explains that there are two Pesukim regarding Hechsher, one regarding Mechubar; the other, regarding Talush, and both are needed (implying that Mechubar is subject to becoming Muchshar)?

Answer: ... that is speaking where the fruit fell into water that was Mechubar, which becomes Muchshar when it is removed from it ...

Implied Question: ... even though they are then Talush (so why is it not obvious)?

Answer: It nevertheless needs to teach us that it is Tamei, even though the initial connection was made when the water was Mechubar.

Question: Why did Rava think that Rav Nachman was pushing him off, bearing in mind that he already learned that from the Mishnah in Machshirin?

Answer: There is a difference between Toveling one's entire body and Toveling one's hands, in that the former is more stringent.

Extention (Part 1): ... and one also needs to answer like this shortly, where the Gemara queries Rebbi Yochanan from a S'tam Mishnah, and answers that even Rebbi Yochanan concedes by Shechitah (that one does not need Kavanah).

Question: Why does the Gemara not query Rebbi Yochanan further from the Mishnah in Machshirin (4:6)?

Answer: The answer must therefore be that washing the hands is different.

Clarification: This means that we do not need Kavanah to permit the animal to be eaten, though we do need the intention to cut the Simanim.