You should be increasing two stitches with the yfwd (AKA a yarnover), and decreasing by two with the part you’re wondering about.
You’re doing a regular K2tog through the back loop, which leaves the finished stitch on your right hand needle as always; one stitch decreased. Slip that stitch back to the left hand needle. You’re then going to move one stitch to the left of it and pick that stitch up and over the first stitch (the one you just worked and slipped back); one stitch decreased. It’s just like you’d do if you were binding off, only backwards. You now move the worked stitch back over to the right hand needle with the rest of the worked stitches.