optical illusion dance

The song is officially German and is called "Hiss - TANZ" and after some checking it may be that there is a heavy German influence in the Cajun music I have heard. That is probably why I originally thought it was Cajun. Turns out the most prized accordions in Cajun music are German made. However I doubt the Germans play the washboard, I could be wrong. We've been getting educated since I posted this.
Getting more info on this all the time from the great responses. Turns out this dance style is called Strumpfhosentanz and if you do a google search of that and hit videos you can find other videos of people doing it. I still think these girls did it best though.