Dogs’ hind legs move when they are scratched in certain spots for the same reason a person get shivers when someone touches his back or shudders when fingernails are scraped down a chalkboard.
It’s a reflex. Some experts say it’s a method that dogs’ wild ancestors once used to scare away predatory animals looking for a dog-sized lunch.
Others say it exists to keep a dog from drowning, when nerves get stimulated on the sides of its body, a dog begins to “paddle.”
Whatever the original reason, this reflex has provided hours of entertainment for some pet owners, though it isn’t much fun for the dog.