You bring the yarn forward (like you're going to purl) then slip the two stitches then move the yarn back and knit two more stitches. The two slipped stitches will not be knit on this row.
Also, how do you sl1 wyif at the end of a row? Am I suppose to k1 at the end even thou it does not say so? I just don't know how to bring the yarn forward without knitting a stitch after it.
As I read it you knit the last stitch, bring the yarn forward, slip the stitch then turn your work to start the next row.
What are you making and do you have a link? More info is always better when asking questions.