On the following rows, you knit the sts you slipped before and slip the sts you knit before. So over 6 sts, k, sl, k, sl, k, sl; the next row is also k, sl, k, sl, k, sl. This is double knitting. You slip the sts from one needle to the other as if you were going to purl, but leave the yarn in back. It takes a few rows for it to look right.