הלכה על זבחים 65:10

Not to sever a fowl sin-offering: That the priest not sever the head from the fowl that comes as a sacrifice — and that is what is called the fowl sin-offering — when he cuts (yimalek) it, as it is stated (Leviticus 5:8), “and he shall malak its head across from its nape, and he shall not sever.” And the understanding of melikah (Rashi on Zevachim 65a) is that the priest plants his fingernail across from the nape — which is the bone that is called the neck-bone — and cuts the bone with his fingernail until he reaches the benchmarks, and [then] cuts the benchmarks (the esophagus and the trachea) with his fingernail, or the majority of one of them. And this is the slaughter of the fowl sin-offering. And the priest needs to not cut it all completely until the head be severed from the body. And about this is it stated, “and he shall not sever.” We have already said in the commandment of building the [Temple] (Sefer HaChinukh 95) that we do not have the ability — nor does one whose “small finger is thicker than our loins” — to find an argument about the details of the sacrifices even from the angle of its simple understanding. And it is enough for this work of ours to make known a little explanation about the content of the sacrifices more generally from the angle of the simple meaning. And I have already written above (Sefer HaChinukh 95) that which I have known and heard.