I think it’s just a really sensitive spot, and they stretch into it the way you’d lean into someone poking a tight spot on your shoulder. It’s likely that it’s also easy to over-stimulate it, though, which is why cats get so grumpy if you do it for prolonged periods.
Cats also can’t scratch this spot on their own. They can clean and groom it (unless u have a Very Fat Kitty), but vigorous licking and nipping aren’t the same as a good scratch. It’s kinda like tickling. Even if you like being tickled, it’s still really easy for it to become Too Much.
There are a lot of nerves there and for a good reason (actually several)! Like most baby mammals, newborn kittens have to be stimulated by their mother in order to pee and poop. Their mother does this by licking around the base of their tail, including the top. It’s a good idea to develop those nerves quick so they can have that ‘whoops, gonna pee!’ response.
For intact females, this is also a breeding behavior. When presenting to a male, an intact female lifts her butt in the air and swishes her tail to the side (and sometimes kneads with her back paws). There’s actually a gland at the base of the tail called the caudal gland.
There are also scent glands in the area, so elevator butt can also be an attempt to exchange odors with their favorite human.
So there u have it. The many reasons for Elevator Butt.
We all needed to know about elevator butt.