I shaped it. But was tricky about it:
I started with pedestal work (trkman's method of teaching heel) to teach my dog to swing her butt around. Once they were swinging reliably into me and "glued" to my leg when I moved, I removed the pedestal. Then I would just kind of fake them out, make them think we are doing a pivot but in reality step my leg out and they just magically slide backwards. From there it's all about reward placement.