You can knit the yarn over through its front loop (or front leg) with the purpose of creating a hole. Let’s see how.
1) Knit until you reach the YO made on the previous row.
2) Insert the tip of your right-hand needle on the front loop of the YO, as you would do to knit any other stitch.
3) Knit normally.
4) Finish the row. This is how it looks from the backside.
This is useful, paired with a decrease, to do a simple buttonhole (1 stitch long). I used this technique to create the buttoholes of my TRAFUL Easy Vest.
The knitted YO is also extensively used in lace knitting. When paired with a decrease, the number of stitches can be kept unchanged, as in my Feather and Fan Beginner Lace Scarf.