We've been talking about the way animals are slaughtered to make sure they're kosher (fit). The way a bird offering is killed to make sure it could be offered in the Mishkan or the Bet HaMikdash is that the Kohen (priest) would take his right thumbnail and press it into the back of the bird's neck, which would sever its neck bone. If the Korban (offering) was an Olah (burnt offering), he would cut both the windpipe and the food pipe. If the bird was a Chatat (sin offering), then only one of those pipes would be cut. Shechita (ritual slaughter) is not effective on these bird Korbanot. Melikah (pinching) is the only way to make them ready for the sacrifice.
We'll compare and contrast the laws of Melikah and Shechita in this Mishnah. The Mishnah says that if somebody does Shechita from the sides of an animal's neck and then cuts the pipes as he's supposed to, the Shechita is valid, and he's permitted to do this in the first place. But if somebody does Melikah from the sides of the neck, the Melikah is invalid. Melikah has to be done only from the back of the neck. The Pasuk (verse) says it has to be done from the back of the neck; it has to be done in the back of the neck. It's the only way that the piercing of the Kohen's nail is acceptable.
If somebody does Shechita on an animal from the back of the neck, the Shechita is invalid because the neck bone is being cut, and then the pipes are being cut. Once the neck bone is cut, the animal is automatically a Terefa. A Terefa is an animal that has physical defects that say it's going to die from those defects, so a Terefa is never allowed to be eaten, even if Shechita is done correctly on that animal. So, in this scenario, when Shechita is done from the back of the neck, the neck bone is severed right away, and then the windpipe and food pipe are cut, so the animal has a broken neck and is therefore a Terefa. Then the Shechita, the cutting of the pipes, happens, which means it happened after the animal was a Terefa, and therefore it's invalid and the animal is unkosher.
If somebody does Melikah from the back of the neck, the Melikah is valid—not only valid, but that's the way it's done, like we stated beforehand. If somebody does Shechita from the throat, the front of the neck, the Shechita is valid, and that's the way it's usually done. If somebody does Melikah from the throat, the Melikah is invalid, like we've stated; it can only be done in the back of the neck.
And here's the summary of the Mishnah: The entire back of the neck is fit for Melikah, and the entire throat, with the sides, is fit for Shechita. What we come out with is that whatever is fit for Shechita—the front of the neck and the sides of the neck—is unfit for Melikah. And whatever is fit for Melikah, which is the back of the neck, is invalid for Shechita.