TT #25 · Parshas Shemini · Aliyah 2 · Passuk 20
Chabad.org — Vayikra Chapter 9 with Rashi
English: “The fats upon the breasts”: Amemar said in the name of Rav Pappa that one should not press the kidney fat onto the meat immediately after cutting, before the meat has cooled; after it has cooled, this is permitted.
Chullin 8b, Tosafot.
E-Daf.com — Chullin 8b (Tsurras Hadaf)
Halachic application — the placement of the fats on the breasts is used to establish the practical timing of the cutting and placement.
The fat around the kidneys should not be pressed onto other meat while the meat is still hot, because the fat can melt into it; the restriction does not apply after the meat has cooled.