There are 2 such spots on cats’ backs.
One is lacated between the shoulders, at the base of the neck. That is about where mother cats pick up their kittens to move them. Because of this, massaging this spot often seems to relax comfort cats. It’s also the spot where you are instructed to put long-term flea medication. Because it’s a spot where the cat can’t groom themself, and thus remove (and ingest) the medication. And many cats enjoy being scratched or petted here – I think because of that, it’s a place they can’t scratch themself. (Horses have a similar ‘itch’ spot in an equivalent spot, at the base of the mane.)
The second spot is the one others identified, at the end of the back just before the tail. I believe some research indicates that the reaction here is a sex-related factor. And personally, I have noticed that they don’t seem to react so strongly here until the cat is sexually mature – kittens don’t seem to have that strong reaction here, for example.