2,494. Checking the Knife

Hilchos Shechita 1:23

A shochet must check his knife at the tip and on both sides, as follows: he passes it and draws it back over the flesh on his finger, and he passes it and draws it back over his fingernail on the tip and on both of its sides to see that it is completely unblemished. After that, he may slaughter with it.

Hilchos Shechita 1:24

The knife must be inspected in the same way after slaughtering with it because if a blemish is found on it afterwards, the animal might be a neveila. This is because maybe the knife got blemished cutting the skin so that when he cut the signs, he did so using a blemished knife. This is why one who slaughters many animals or birds must check the knife in between uses. If he didn’t check in between, but he checked after the last one and found the knife blemished, there’s a doubt about all of the animals or birds; even the first ones might be neveilas.