My cat loves to have his back scratched, especially an inch or two in front of his tail.  He would get the most contented look on his face.

Over the last month or so, he has started licking and chewing the fur around his shoulders every time I scratch.  I noticed that his shoulder muscles twitch when I scratch now.

WTF is up with that?  His mother does the same thing, but only when you scratch right at the base of the tail.

You hit the ass button!

Kitty Cats

Muscle spasm/twinge from hitting the nerve bundle at their tail.

Kitty erogenous zone.