Chullin, Chapter 1, Mishnah 2. The first mishnah of the chapter taught us who is fit to perform shechitah. This mishnah turns to the instrument: what happens when the slaughtering was carried out not with a proper knife, but with something else entirely.
A sickle, a stone, a reed
The mishnah begins: "Hashochet bemagal yad" - one who slaughters with a hand sickle. A sickle of this kind has two working sides, one smooth and one serrated. Were he to use the serrated side, the case would be obvious: the teeth would tear the simanim of the animal rather than cut them, and torn simanim disqualify the shechitah on the spot. Our mishnah is therefore describing a person who used the smooth side.
The same applies to "betzor" - a sharp stone - and "uvekaneh" - a sharp reed. In all these cases, "shechitatah kesheirah," the shechitah is valid.
Note the wording. The mishnah says "hashochet," one who has already slaughtered, which tells us that we are speaking after the fact. Bedi'eved the animal is permitted; lechatchilah a person should not be reaching for such instruments at all.
Two limitations apply. To begin with, the stone and the reed have to be objects that stand on their own, separate from the earth. A stone that someone uprooted and afterward pressed down again into the soil still counts as separate, since its connection to the ground is no longer natural; but if the shechitah was done with an item truly joined to the earth, it does not count. Beyond that, the sickle case refers only to the side without teeth. Strictly speaking that side severs the simanim properly, yet the Chachamim had a worry: once the smooth side is sanctioned, people will drift into using the toothed side as well. So they gave their approval only after the fact, when the animal had already been slaughtered.
"Hakol shochatin" - everyone may slaughter
"Hakol shochatin." Anyone may perform shechitah, including a Jew who does not conduct himself according to the laws of the Torah, a mumar, one who chases after the pleasures and the conveniences of this world even at the cost of transgressing the word of Hashem. We may assume that such a person slaughtered correctly, since the act of shechitah itself is not especially difficult to carry out.
There is, however, a condition: a reliable person must stand over him and verify that the knife is smooth both before and after the slaughter. A mumar cannot be trusted to examine the blade, and the knife must be inspected after every single shechitah. So someone trustworthy has to establish that the knife is fit for use.
All of this concerns a mumar of that sort. It does not include a Jew who desecrates Shabbat befarhesya, in public, or who serves avodah zarah, or who transgresses the majority of the Torah. The shechitah of such a Jew is problematic, on the level of the shechitah of a non-Jew.
"Ul'olam shochatin" - any time, any place
"Ul'olam shochatin." Shechitah may be performed at any time, even at night in the dark, provided that there is a lamp there giving light.
And in any location: a person may slaughter while standing above, on a rooftop, or on board a ship. There is no worry that a bystander watching someone slaughter at a height will conclude that the animal is meant for the sky or for the constellations. So too on a boat: although the blood spills out into the water, we do not suspect the slaughterer of paying homage to the sea, since anyone can see that he is aiming the blood over the side to keep the deck clean. Still, as a general rule a person may not arrange his shechitah so that the blood pours straight into water, a subject the mishnah will address in the coming perek.
"Uvechol shochatin" - and with what may one slaughter
"Uvechol shochatin." One may slaughter with any sharp blade. A knife is not required; glass will do, or anything else sharp enough to cut.
The mishnah then lists the exceptions: "chutz mimagal katzir, vehamegeirah, vehashinayim, vehasiporen." Not with a harvest sickle, not with a saw, not with teeth set in a jawbone that has been removed from the animal, and not with an animal's nail while it is still attached to the animal. The reason given is "mipnei shehein chonkin," because these tear the simanim.
With the harvest sickle, the saw and the jawbone (that is, the initial three items on the list), the danger is mechanical: the points can snag the simanim and rip them apart rather than slice cleanly, and a ripped siman renders the shechitah void of itself. The nail is disqualified for an altogether different reason. Shechitah may not be performed with anything joined to a live animal; the implement must be free standing, neither growing from the ground nor part of a creature. The nail is therefore ruled out only for as long as it sits on the animal, and once it has been separated it may serve as a slaughtering tool. The jawbone never becomes usable, since its teeth and ridges give it the character of a serrated blade, and such a blade tears instead of cutting.
Pushing and pulling
Consider how a harvest sickle is built. Its teeth all angle back toward the handle. If a person extends the sickle and draws it back toward himself, he is cutting with those teeth, and they will tear the simanim open, rendering the animal a neveilah and forbidden as food. But if he holds it close and pushes it away from himself, the blade produces a smooth cut and the teeth do not engage the simanim at all.
Here too the Chachamim were troubled. They ruled out the sickle altogether, forbidding it even when moved away from the body, since they feared it would in time bring a person to draw it toward himself, which turns the animal into a neveilah. And they went further still: an animal actually slaughtered with a sickle in the outward motion is forbidden as food. Whether it also carries the tum'ah of a neveilah is a matter of disagreement.
"Hashochet bemagal katzir bederech halichatah" - one who slaughtered with a harvest sickle by pushing it forward, so that the cut came out smooth, but who used a keli that was not permitted to be used.
"Beit Shammai poslin." Beit Shammai rule that the shechitah is invalid by rabbinic law, and further, that the animal transmits tum'ah.
"Uveit Hillel machshirin." According to Beit Hillel the disqualification reaches only so far: eating the animal is prohibited, yet no tum'ah attaches to it. Beit Hillel did not wish to widen the scope of tum'ah for Bnei Yisrael.
When the teeth are filed down
"Ve'im hecheliku shineha" - and if the sickle's teeth were smoothed out, rubbed against something until the serrations were gone - "harei hi kesakin," it has the status of an ordinary slaughtering knife. It may then be used in either direction, pushing or pulling, and the shechitah is kosher either way, because it is now simply a knife.