Hullin, Chapter One, Mishnah Two
This mishnah continues to provide general rules with regard to slaughtering including the instrument with which one slaughters. We should note that modern halakhah dictates that Jewish slaughter must be done only with an extremely sharp knife, one designed specially for such a purpose. The Mishnah still reflects a time when Jews used other instruments as well.
1) If one slaughtered with [the smooth edge of] a hand sickle, with a flint or with a reed, the slaughtering is valid.
2) All may slaughter; at all times one may slaughter; and with any implement one may slaughter, except a scythe, a saw, teeth or a finger nail, since these strangle.
3) One who slaughtered with a scythe, moving it forward only:
a) Bet Shammai declare it invalid,
b) But Bet Hillel declare it valid.
4) If the teeth of the scythe were filed away it is regarded as an ordinary knife.
Section one: All of these instruments have smooth cutting surfaces and therefore can be used to slaughter.
Section two: This section summarizes that which we have learned up until now. It then adds a caveat concerning instruments that cant be used. These instruments do not sever the trachea and esophagus, as is required, but rather tear them out, thereby causing strangulation. An animal slaughtered with one of these instruments is considered a nevelah.
Section three: The scythes teeth are bent back, pointing only in a backwards direction. Therefore, if one slaughters by moving the scythe forward, the teeth wont tear the neck and the neck will effectively be sliced. Neverthless, Bet Shammai declare it invalid lest he bring the scythe back and tear the neck. Bet Hillel declare the animal valid.
Section four: If the teeth are filed away, then the scythe can be treated as a normal knife and one can use it for slaughtering.