I recently read in Wikipedia the explanation of why squirrels, unlike most mammals, descend from trees upside down: "tree squirrels, unlike most mammals, can descend a tree head-first. They do so by rotating their ankles 180 degrees, enabling the hind feet to point backward and thus grip the tree bark from the opposite direction."
After reviewing my photos, I found several photos that show the position of the legs during the ascent and descent, and it is clearly visible that when descending, the claws of the hind legs are turned in the opposite direction, securing the squirrel.

Nikon Coolpix P900

Cropped to make a collage