As the video itself says...read the description!
Ever since she was a kitten, our cat Pumpkin has exhibited strange behavior when the last portion of her back is scratched or rubbed. She caterwauls (though I guess it could be more charitably described as "cat singing" or even "yodeling"), gets a spaced out look on her face and will usually start to lick the air with her tongue. She may also make jerky little movements with her head.
There's nothing obviously odd about her skin, and she has yearly checkups at the veterinarian along with all of her shots. And despite the painful sounding noises, Pumpkin doesn't seem to mind being scratched on her back one bit. I would think that if this were painful, she'd run away, not allow anyone to touch her back or become angry. Instead she starts to purr intensely (which you can just hear at the end of this video).
Pumpkin is (and always has been) a very vocal cat. She makes noises I didn't know that cats could make. She is a little over ten years old.
The spot you see on the video image is a result of condensation on the Handycam's lens. I didn't clean it off.