When you work a sewn edge with a folded hem a whip stitch is used to fasten the live stitches down. You can see how I work this stitch in this video:
With live stitches you will pull the yarn through the stitch, then attach it to the stitch on the knitting below where you want it to lie. Take care to always use the same row of knitted stitches to attach it to.
If you are using whip stitch on bound off stitches you will go into the base of the stitch instead of the live stitch.
If you are using whip stitch on a column of stitch rows you will treat each row the same way as the stitch. This is frequently used to attach the inside of a steek cut such as in this video:
