Masechet Chullin, Perek Bet, Mishnah Gimel discusses the disqualification of Derisah, pressing down: the pipes of the neck were cut by the knife being pressed down instead of with the back-and-forth sawing motion that is halachically needed.
The Mishnah says that somebody cut off the animal’s head with one stroke. He chopped it off with a downward blow, as opposed to a back-and-forth motion. The shechitah (slaughtering) is invalid because the knife was pressed downward, and it has to be the back-and-forth motion. If somebody was slaughtering an animal with the regular back-and-forth motion he was supposed to use and cut off the head with one stroke, either forward or backward, as he was chopping and slicing the neck, he just chopped the whole head off as he was doing it. is if the blade of the knife is a full neck more than the neck of the animal, so the blade’s length is double the width of the animal’s neck. The shechitah is valid because a blade of that size could cut the pipes with a single sawing stroke, and we don’t make the assumption that it was pressed down on.
But if the blade is shorter than that, we assume that it would have been impossible to slice the pipes in a single stroke, and thus he pressed down on them. In another case, he cut the heads of two animals with one stroke. If the knife is a full neck’s width more than the width of the necks of the two animals, a knife of that size could cut both necks with a single sawing motion, and there would have been no need to press down on the neck. But if you were dealing with a shorter knife, we assume that it wouldn’t have been possible to sever the pipes with a single stroke unless he pressed it downward. Therefore, the fact that it happened here with one stroke means he pressed it downward.
Again, with the shorter knife, when do these halachot (laws) apply? When do we say that one neck width more than the necks of the animals is the minimum? Only when the heads of the animals are being sliced off in a situation where he moved the knife forward and not backward, or backward and not forward. In other words, the shechitah was completed with one stroke. But if he moved back and forward—he moved the knife in two directions—we say that even if it was a very short blade, a little tiny—I mean, a little tiny scalpel—then the shechitah would be valid, because the double stroke, forward and backward, has the ability to cut the pipes without pressing down, even though we’re dealing with a very small knife.
A knife fell by itself, without the force of a person. Not that he threw the knife, not that he pushed it downward and it went on its own, but a knife fell by itself and slaughtered an animal. Even if it is slaughtered in the proper way, with the sawing back-and-forth motion, the shechitah is invalid. The Pasuk (verse) tells us, “you should slaughter and you should eat,” which tells us, “whenever you slaughter, you’re allowed to eat.” Only when a person does the shechitah is he permitted to eat the food. Only in that type of situation is the shechitah kosher (valid).
If a knife fell out of the hand of the Shochet (ritual slaughterer) after he started to cut, and then he picked it up and continued cutting the neck; or if his clothes fell off after he started to cut, and then he picked them up and continued to cut; or, in the third case, if he sharpened the knife and became very tired, then began to slaughter, but was so tired he didn’t have the ability to continue, and his friend came and continued to slaughter whatever had to be done—if he paused for the amount of time it takes to slaughter an animal, then the shechitah is pesulah (invalid). The shechitah is invalid, so the pause he takes has to be a very short amount of time. It says that even if he paused for the amount of time it takes him to examine the knife to make sure that it’s smooth, which is a shorter amount of time than the whole shechitah, then the shechitah is invalid.