The reason you shouldn't pinch a cat's back is mainly because the area between a cat's back and neck is a sensitive area, and this area is also one of the cat's weapons. Strong stimulation may cause a painful reaction.
A cat's spine has many sensitive nerve endings, which produce a pain response when strongly stimulated. Furthermore, this area is also one of a cat's weapons. In the wild, when a cat feels threatened, it will arch its body and raise the fur on its back to appear more imposing and larger, thus scaring away enemies.
