When you knit, your yarn is in back, right? For a yarn forward, you'll bring your yarn forward as if you are going to purl. When you do the next stitch, in this case k2tog, your yarn will wrap around your needle and give you the eyelet and an increase at the same time. The extra stitch is offset by the k2tog, so you'll come out even in the end.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog