Rashi on Leviticus 5:8/רש"י על ויקרא ה׳:ח׳

1והקריב את אשר לחטאת ראשונה AND HE SHALL OFFER THAT WHICH IS FOR A SIN-OFFERING FIRST — This verse establishes the general rule that the sin-offering is always sacrificed before the burnt-offering. — To what may this be compared? To the case of an advocate (in our case the חטאת) who went in to the king to obtain pardon for his client. When the advocate has gained that pardon, then the present (עולה) is brought in after him (Zevachim 7b).

2ולא יבדיל [AND HE SHALL NIP ITS HEAD …] BUT SHALL NOT SEPARATE IT — i.e. he nips only one organ (either the gullet or the windpipe) (Chullin 21a).

3עורף is the surface of the head which slopes down towards the neck);

4‎‏מול עורף is the part bordering on it which sees the עורף (the part next to the עורף and within sight of it) — that is, the length of the whole back of the throat (Sifra, Vayikra Dibbura d'Chovah, Chapter 18 7: Chullin 19b).