Start purl row using circular needles while knitting flat

I am using circular needles to knit a flat project. I cannot figure out how to start a purl row.

Check out the demo of a small project video, in the video she is knitting flat on circs…
http://www.knittinghelp.com/videos/knitting-tips

You turn to the back just like you do with straight needles. If you slid your work to the middle of the cable you may need to slide it back to the end of the needle, but otherwise it’s the same. The working yarn will be at the end just like straights.

Just turn like you do at the end of a straight needle and purl back along the stitches.