That's an old myth. If you run a snake over with a car or even chop it into pieces, it keeps writhing around (even the pieces keep moving for quite a while, often well over an hour, even if they've been skinned and gutted). People thought that snakes had some amazing ability to stay alive, or that the pieces were still alive and could spawn new parts. Presumably this is what a few people thought when they whacked or cut up snakes and then left them, and the myth was born. Some other people probably tried to check it out and kept hacked up snake bodies, saw that they were writhing, but then the next morning they were still, and figured they'd just assume that whatever they guessed was fact (as is human nature for some reason I still can't understand). It might have been a bit different, but I'm sure that these myths came from peoples' observations of dead snakes' bodies continuing to move around for long periods of time after death.
