The tickle spot that makes them paralyzed and twitch?

My past dogs, 1 was on his spine, if i scratched it hard, he would not move, his neck would stretch out, but his leg would twitch in mid air like a flea scratch.... it was hysterical.
My current dog i found it with tickle pressure on the back of her neck and an inch behind the ears, the leg just flaps, her neck goes out, it almost seems like im tormormenting them, but they dont complain.... anyone know what this behavior is? Do they actually enjoy it? or is it like being held down and having someone tickle you?

My cat is itchy, too. When I scratch her armpit she does this Stevie Wonder thing with her head. It used to scare people. =)

I think they are just itchy. It's probably like that place in the middle of your back where you can never scratch. I'm not sure. My cat's never had any problems.

I think it's a nerve thing.. I don't know. All my cats and my parents dogs do this.. it's pretty cute!! :D