Gemara

(Mishnah): If one paused during Shechitah to pick up the knife or his clothes that fell, or to sharpen the knife, or he grew weary and someone else completed the Shechitah, if the delay was the time needed for Shechitah, the Shechitah is invalid;

Question: What is the time for Shechitah?

Answer #1 (Rav): It is the time to slaughter another animal.

Question (Rav Kahana and Rav Asi): (For a bird,) is it the time to slaughter an animal, or the time to slaughter a bird?

Answer #1 (Rav): For an animal, it is the time to slaughter an animal. For a bird, it is the time to slaughter a bird.

Answer #2 (Shmuel and R. Yochanan): Also for a bird, it is the time to slaughter an animal.

(R. Chanina): It is the time to bring an animal and slaughter it.

Objection: If so, the time varies depending how far we must go to find one!

Answer (Rav Papa): They argue about the time to cast the animal to the ground;

i. (R. Yosi b'Ribi Chanina): It is the time to pick up an animal, make it crouch down, and slaughter;

i. For a large animal, it is the time for a large animal. For a small animal, it is the time for a small animal.

(Rava): If one slaughters with a blunt knife, even if he spends the whole day slaughtering one animal, it is valid.

Question (Rava): Do small pauses join up? (I.e. the total delay is the time for Shechitah.)

Rava could not resolve this from his previous law. There, he was constantly cutting. He did not pause.

Rishonim

Rif (7a): (Gemara) Rav says that for an animal, the time for Shechitah is the time to slaughter an animal. For a bird, it is the time to slaughter a bird. Shmuel and R. Yochanan say that also for a bird, it is the time to slaughter an animal. R. Yosi (b'Ribi Chanina) says that it is the time to pick up an animal, make it crouch down, and slaughter. The Halachah follows Shmuel and R. Yochanan. They are a majority against Rav.

Rambam (Hilchos Shechitah 3:3): The Shi'ur of Shehiyah for a small animal is in order to lift a small animal, lay it down and slaughter it. The Shi'ur for a big animal is in order to lift it, lay it down and slaughter it. For a bird, it is the time to lift a small animal, lay it down and slaughter it.

Rosh (2:9): Rashi's text says that Rav Papa explains that R. Yochanan and R. Chanina argue about whether we include the time to lay the animal on the ground. This is wrong. Rav Papa only came to answer that the time does not depend on the distance to the nearest animal! He teaches that we discuss when there is an animal in front of us. Rashi rules like Rav, because R. Yosi holds like him; the Shi'ur for a small animal is based on a small animal. It follows that the time for a bird is the time for a bird. Perhaps he rules like R. Yosi only regarding what Rav and R. Yochanan argue about, but not regarding the time to lift and lay it down. Rav, Shmuel and R. Yochanan do not include this. We follow them against R. Yosi . However, in the primary text, Rav Papa merely answers that the animal is in front of us. It seems that Rav, Shmuel and R. Yochanan did not discuss the time to lift and lay it down. They discuss only the time for Shechitah. R. Chanina explained what they did not discuss. He explains the Mishnah; he does not comment on them. No Amora disagrees with him. The Rif connotes like this. He rules like Shmuel and R. Yochanan, and brought R. Chanina's law. Since he rules that the Shi'ur for a bird is based on an animal, surely the time for a small animal is based on a big animal! Rather, he brought R. Yosi to teach that we include the time to lift it and lay it down. Why didn't he bring R. Chanina, who also says so? This would have been better, for the Halachah does not follow all of R. Yosi words! It is because R. Yosi explained better the Shi'ur of lifting and casting down. Does the time for a bird include the time to lift and lay down an animal, or is it just the time to slaughter an animal? Presumably it is only the time to slaughter; we are not so lenient. This is why the Rif brought R. Yosi , for he said that the time for a small animal is to lift it and lay it down and slaughter it, (the time) for a small animal is based on a big animal, and for a bird is based on a bird. Even though the Halachah is unlike him regarding the time for Shechitah itself (he bases everything on the time for a small animal), the Halachah follows him regarding the time to lift it and lay it down. R. Chanina connotes like this. He said 'the time to bring another animal', i.e. a similar animal. He did not say 'the time to bring an animal and slaughter.'

Ran (7b DH Dakah): R. Yosi explains Rav, Shmuel and R. Yochanan. The time for Shechitah includes the time to lift it and lay it down. Therefore, the Shi'ur for an animal is to lift it and lay it down, i.e. a small animal for a small animal, and a big animal for a big animal. The Rambam rules like this. Rashi says that just like we distinguish between big and small animals, we distinguish between birds and animals. Therefore, we hold like Rav, who says that the time for a bird is that for a bird, for R. Yosi holds like him. However, the Ge'onim, Rif and Rambam say that R. Yosi does not discuss a bird at all. Therefore, we hold like Shmuel and R. Yochanan against Rav. Is the Shi'ur for Shechitah based only on the Simanim, or also to cut the skin? Is it the time to cut the entire Simanim, or the majority? Presumably it is the time to cut the majority, for mid'Oraisa this is enough l'Chatchilah. Surely we include the time for to cut the skin, for the tradition is for the time for a normal Shechitah, which begins with the skin. Some say that the time is to cut the entire Simanim, for Shechitah applies to all of them.

Rosh (ibid.): The Rambam said that for a bird, it is the time to lift and lay down a small animal and slaughter. This sounds like a compromise. This is difficult. Since we are lenient to allow for a bird the time to lift and lay down an animal, we should be lenient to allow the time for a big animal! Also, since he allows for a small animal only the time to lift and lay down a small animal, but not of a big animal, why are we lenient for a bird to allow the time to lift and lay down an animal?

Beis Yosef (DH veha'Rosh): The Rambam did not compromise. He holds like his Rebbi, the Rif. It is difficult to say that the Rif brought R. Yosi just regarding to lift it and lay it down, but not for Shechitah. The Rosh asked why we are more lenient about a bird than for a small animal. Had we said so ourselves, this would be difficult. However, the Rambam holds that the Amora'im said so. They knew that a bird's life-force is like that of a small animal. (I.e. it can survive a similar pause during Shechitah.) Alternatively, this was a tradition from Moshe from Sinai, like the Rashba says.

Rambam (4): If one paused enough to lift, lay it down and slaughter the minority of the Simanim, but not a full Shechitah, it is Safek Neveilah.

Question (Rosh ibid.): If one paused enough to lift, lay it down and do a full slaughter, it is Vadai Neveilah! Do not say that it is Vadai only if he paused long enough to totally cut the Simanim. It suffices to cut the majority! We are lenient to allow the time to lift it and lay it down only because one cannot slaughter without this. However, the majority of the Simanim is a Kosher Shechitah.

Beis Yosef (YD 23 DH v'Od): The Rosh's text of the Rambam was mistaken. However, even our text is difficult. Since the pause was not enough for a full Shechitah, why is it a Safek? It should be Vadai Kosher!

Poskim

Shulchan Aruch (YD 23:2): If one began to slaughter, and lifted his hand before finishing, and paused, whether he was Shogeg, Mezid, Ones or wanted to do so, and he or someone else finished the Shechitah (Rema - until the majority of the Simanim), which is the Shi'ur for Kosher Shechitah, it is Pasul.

Shach (2): The same applies if he did not lift his hand. This is just the normal case.

Shulchan Aruch (ibid.): For a small animal, the Shi'ur is to lift a small animal, lay it down, and slaughter it. For a big animal, the Shi'ur is to lift a big animal, lay it down, and slaughter it. For a bird, the Shi'ur is to lift a small animal, lay it down, and slaughter it. Some say that it is the time to slaughter the majority of one Siman of a bird, without lifting it or laying it down.

Beis Yosef (DH Aval): The Ran holds that since R. Yosi did not mention birds, this shows that he holds that there are only two Shi'urim, of big and small animals. The Shi'ur for a bird is based on a small animal. Also the Rambam says so.

Taz (1): The Shulchan Aruch is like the Rambam. The Tur says that also the Rif and Rosh allow for a bird or small animal the time to lift and lay down a big animal and slaughter it. The Drishah refuted this. I say that the text of the Tur should say that for a bird, the Rif agrees with the Rosh that we allot the time to slaughter a big animal, but the Rif argues and does not allow time to lift and lay it down.

Gra (2): The latter opinion holds like Rashi. (Likut - It mentions a bird, for regarding a bird it argues with the first opinion about two matters, the time to lift it and lay it down, and the time of Shechitah itself. However, the coming Likut explains simply, that the latter opinion argues only about birds.)

Gra (4, Likutz): Rashi, Semag and the Mordechai do not include the time to lift it and lay it down even for animals. The latter opinion is the Rosh, who says that only for birds we do not include the time to lift and lay down. The Rosh says that the Rif rules like R. Yochanan, and gives (for Shechitah) even for a bird the time for a big animal, and all the more so for a small animal. However, the time to lift it and lay it down is according to the size of the animal, and it is not given at all for birds. The Ran explains the Rif like the Rambam; this is the first opinion in the Shulchan Aruch.

Shulchan Aruch (ibid.): According to this opinion, after beginning to slaughter a bird and cutting a little until the blood exudes and he lifted the knife from the neck, he must not complete the Shechitah, lest he already cut part of the Veshet. Even if he lifted the knife only a little, we are concerned, for the time to slaughter a bird is very small. The majority of one Siman is slaughtered quickly. Even if the Shochet is sure that he cut only the skin, we do not rely on him, since blood exuded. If he asks what to do after he lifted the knife, we tell him to cut only the Kaneh in another part, and afterwards turn the Veshet inside out and check (for a drop of blood, which would show that it was cut). In practice, one should be stringent like this opinion, unless it is pressed circumstances or a great loss. Then, one may rely on the first opinion.

Shach (6): We are not skilled to check, so it is Tereifah. The Maharshal says that to avoid a big loss or for the need of a Mitzvah, one may be lenient.

Gra (5): A pause Mashehu (any amount) disqualifies in the first half of the Veshet.

Rema: Our custom is to disqualify any pause, even Mashehu, for a bird or animal. One should not deviate. If after Shechitah a reed or similar matter was found object on the vok, and it was slaughtered with it, it is Tereifah, for surely he delayed Mashehu to cut it after cutting the Siman.

Taz (2): This is only if he lifted the knife even Mashehu. Since he removed it from the neck, we do not distinguish between a big or small pause. If he did not lift the knife at all, just he pushed it from place to place, even we are Machshir, like we find regarding Shechitah in two or three places (21:3). The Yam Shel Shlomo (2:15) says that even if his hand slipped below, it is not considered Shehiyah. It is Kosher even if the Veshet was punctured, for it is all one Shechitah.

Shach (7): We are stringent even if it will be a big loss. Darchei Moshe, the Bach and Acharonim say so, unlike the Levush.

Gra (7): Since we do not know the exact Shi'ur, they agreed to disqualify in every case.