Chabad.org — English translation + Rashi
Torah Temimah points to Chullin 11a.
E-Daf.com — Chullin 11a
HebrewBooks / Shas PDF
English: “Opposite the backbone” — what is atzeh? The place where the kidneys counsel.
Definition of atzeh — the alyah is removed opposite the end of the spine where the kidneys meet.
The alyah is removed opposite the end of the spine where the kidneys meet. The Gemara explains “atzeh” as the place at which the kidneys “counsel,” i.e., the region near the spine where the kidneys are situated. The practical consequence is a precise anatomical landmark for the separation of the complete alyah.