This technique is used to shape your knitting on the purl side by removing stitches from your work. It compliments the k2tog decrease and mirrors the ssp decrease.
Step 1: Purl the next stitch in the row.
Step 2: Slip the stitch back onto the main needle.
Step 3: Slip the second stitch over the first stitch (like a pass over slip stitch but reversed).
Step 4: Slip the stitch back to the working needle.
Step 5: Wrap the working yarn over your index finger.