Gemara

(Beraisa): A knife with many nicks is (Pasul) like a saw If a knife has only one nick, if it is Ogeres, it is invalid. If it is Mesuchseches, it is valid.

(R. Elazar): Ogeres is a nick with Uktzim (places for the Siman, i.e. Kaneh or Veshet, to get caught) on both sides. Mesuchseches has an Oketz on only one side.

Question: Presumably, Ogeres is invalid because the first Oketz is tears the flesh, and the second tears the Siman;

Even Mesuchseches should disqualify Shechitah! The pressure of the knife tears the flesh, and the Oketz tears the Siman!

Answer: The nick is at the end of the knife.

Objection: Still, when he is Molich (slides the knife away from himself while cutting), this cuts the flesh. When he is Mevi (returns the knife), this tears the Siman!

Answer: He was only Molich. He was not Mevi.

(Rava): If a knife has an Ogeres nick, he may not slaughter with it. If he did, the Shechitah is invalid;

If it has a Mesuchseches nick, he may not slaughter with it. If he did, the Shechitah is valid.

Question (Rav Huna brei d'Rav Nechemyah, to Rav Ashi): You said that Rava said that a Mesuchseches nick disqualifies the Shechitah. Here, he is Machshir!

Answer (Rav Ashi): If he was only Molich, the Shechitah is Kosher. If he also was Mevi, it is invalid.

Rishonim

Rif (Chulin 5a): Rava is Machshir a Mesuchseches nick if he was only Molich. If he also was Mevi, the Shechitah is invalid

Rosh (1:24): Every Oketz of a nick tears and does not cut, both when it encounters the Siman, and when it descends from it. When a knife has many nicks, we do not distinguish Ogeres from Mesuchseches. Even if all the Uktzim are towards the top, and he was Molich and not Mevi, it is Tereifah. We infer that also when it descends, it tears. When the nick is at the top of the knife, when he began to be Molich, the knife was unable to tear flesh until passing the nick, so there is no concern lest the Oketz touched the Siman. Even so, one may not slaughter l'Chatchilah when there is a nick at the end of the knife, lest he be Molich and Mevi. The Rif said that Rava is Machshir a Mesuchseches nick if he was only Molich. He did not require the nick to be at the end of the knife. He holds that it does not tear when it descends. He explains the Gemara's answer to say that the Oketz is on the side of the nick closer to the top of the knife. It does not tear when the Siman descends into the nick. The Gemara asked that when he is Mevi it will tear, and answered that he is only Molich. Similarly, if the Oketz is closer to the bottom of the knife, it is Kosher if he is only Mevi. However, the Rif should have taught that Holachah alone is Kosher only if the Oketz is closer to the top of the knife, and Mevi alone is Kosher only if the Oketz is closer to the bottom of the knife! Rashi's Perush (the first Perush that the Rosh brought) is primary. The Sugya proves that an Oketz tears even during descent.

Beis Yosef (YD 18 DH u'Mah she'Chasav): One cannot say that the Rif explains 'Klapei the top of the knife' to mean that the edge of the Oketz faces the top. If so, why are we Machshir when he did Holachah and disqualify when he did Hava'ah? We should say oppositely, i.e. it is Tereifah if he did Holachah (for then the Oketz encounters the Siman - PF), and Kosher when he did Hava'ah! Rather, (the Oketz faces the bottom, and) the back of the Oketz is towards the top. Also the Rashba says that the Oketz is not at the top of the knife, rather, it is Klapei the top, and the Oketz leans towards the handle. He agreed to the Rif's Perush.

Rebuttal (Bach 8 and Taz 3): The Beis Yosef's answer is totally unlike the connotation of the Gemara. Rather, every nick has two Uktzim, one towards the top, and one towards the handle. If the latter was sharpened (and smoothed), and there is only the top Oketz, even if it is in the middle, it is Kosher if he did only Holachah. 'Klapei the top' means on the top side. If he does only Holachah, the Oketz descends from the Siman and does not touch it. The Rosh says 'Klapei the top' and 'on the top side' interchangeably. Rashi (Eruvin 5a) explains 'Klapei the head of the Mavuy (alleyway)' similarly.

Ran (top of 5a): The Rif is Machshir when he was only Molich, i.e. he slaughtered in the direction which the nick is not felt. If so, why do we disqualify a knife with many nicks? Their Uktzim do not touch the Simanim! Why is this different than one nick? (We must say that they are Mesuchsechos. If not, even one nick disqualifies! - Beis Yosef YD 18 DH v'Ika.) We can say that when there are many, we are concerned lest he forget, and be Mevi (and not notice - Beis Yosef). This is why Beis Hillel (18a) are Machshir Shechitah with a reaping scythe only regarding Tum'ah, but they forbid eating it.

Rambam (Hilchos Shechitah 1:16): If one checks a knife through Holachah and does not feel a nick, and when he checks through Hava'ah he feels a nick, if he slaughtered only through Holachah, it is Kosher. If he was Mevi, it is Pasul.

Rosh: The Rambam's law is correct. If the Oketz was towards the top, and he did not feel a nick when he was Molich his finger along it, it is known that the Siman will not tear. However, this is difficult. Since on one side (of the nick, i.e. the Oketz) it is vertical, how can he not feel the nick?! Regarding an Ogeres nick, we say that the first Oketz tears the flesh, and the second tears the Siman. This shows that an Oketz tears both on the descent and on the ascent! Likewise, one feels the Oketz of a nick whether his finger descends or ascends!

Bach (9): The Rosh agrees with the Rambam's law, but holds that a nick that is felt in only one direction almost never occurs. R. Yerucham says in the name of the Rashba that Mesuchseches is in one direction, i.e. it is a low Oketz, like those of a reaping scythe. It (the Oketz) is inclined, therefore it is a small descent. Perhaps this was the Ge'onim's intent. The Ge'onim agree that if a nick is felt on the descent, it disqualifies. Presumably they agree that a nick at the end of the knife does not disqualify b'Di'eved, even though it is felt, for surely it passed before the skin and meat were cut.

Poskim

Shulchan Aruch (YD 18:4): If when one checks a knife through Holachah he does not feel a nick, but during Hava'ah he feels a nick, this is called Mesuchseches. If he slaughtered only through Holachah, without Hava'ah, it is Kosher

Beis Yosef (DH v'Keitzad): Rashi explains that 'Ogeres' is an expression of gathering, i.e. it catches the fingernail between its two Uktzim. Ba'al ha'Itur says that 'Ogeres' is related to 'Tachgor'; the fingernail is girded in it. Mesuchseches has only one Oketz; the other was smoothed through a sharpening stone. It is not Ogeres; it only causes the knife to confront the meat.

Gra (8): The Shulchan Aruch rules like the Rashba, who explains that the Oketz is towards the top (and the smooth side is towards the bottom, like the Bach explained - PF). He holds that it tears only on the side of the nick (i.e. if the movement of the knife brings the Oketz to where the middle of the nick was - PF).

Gra (Likut): The Hava Amina was that Mesuchseches is Kosher in every case. We answered that it is Kosher only at the top. The Rashba rejected Rashi's Perush, for the Rashba holds that the first Oketz cannot cut the skin and meat (so even Ogeres should be Kosher). Also, why do we disqualify if there are many nicks? The second is not at the top, therefore it alone disqualifies! Also, Rav Ashi answered that he was only Molich. He did not say that the Oketz is at the top. According to the Rashba, he did not need to say so. Since he said that he was only Molich, we must say that the Oketz is towards the side from which the (top of the) knife passed (if not, it would tear the Siman). This is why the Rif brought only this answer. The Ro'oh and Rosh explain like Rashi.

Shulchan Aruch (ibid.): If he was Mevi, it is Pasul.

Shach (7): We disqualify even if he did only Hava'ah, for the Oketz tears the Siman.

Shulchan Aruch (ibid.): This is if he noticed the nick before the Shechitah. If he found the knife Kosher before Shechitah, and found a Mesuchseches nick, and he is sure that he did only Holachah, the Shechitah is Pasul.

Beis Yosef (DH Kasav): The Mordechai brings this from R. Peretz. If he did not know before Shechitah, anything that is not incumbent on a person, he is not aware of it. (Perhaps he was Mevi a little, but thinks that he was not.) Also Hagahos Maimoniyos citing Semak and the Kolbo say so.

Rema: Some are stringent to forbid every Mesuchseches, unless it is at the very top of the knife if he slaughtered through Holachah, or at the very bottom of the knife if he slaughtered through Holachah.

Gra (10): This is like Rashi.

Rema: Since we are not experts about what is called the top of the knife, we should consider it Tereifah in every case.

Beis Yosef (DH NImtzeinu): If a Mesuchseches nick is felt slightly when the finger descends into it, the Rif and Rambam agree that it is Pasul. They are more stringent than the Rosh. The Rosh is Machshir when the nick is at the top and he was only Molich. The Rif and Rambam consider this like Ogeres, and they disqualify.

Bach (7): Rashi explains that even if an Ogeres nick is at the end of the knife, the first Oketz cuts the skin, and the second tears the Siman. Likewise, if a Mesuchseches nick is in the middle, the sharpness of the blade before the nick cuts the skin, and the Oketz tears the Siman. If it is Mesuchseches at the top and he was only Molich, the knife could not cut the skin and meat before the nick. The same applies if the nick is at the handle and he was only Mevi. However, if the Oketz faces the top, even if it is at the top, if he is Molich forcefully, the knife tears the skin and Siman. The same applies if the Oketz faces the handle, even if it is at the handle, and if he is Mevi forcefully.

Taz (3, Shach 6, Gra 8 and Drishah): Here, Holachah and Hava'ah are not when we hold the knife in one hand with the blade up, and we are Molich the finger along the blade. If so, this Sa'if does not make sense! Rather, he is Molich the blade along the finger. Hava'ah is similar. Sa'if 9 explicitly says that this is how they checked.