It's probably like when you knit mittens and you need to leave a hole for the thumb to be worked on after the hand is done. I have a picture here
that shows how I just pulled a longish piece of yarn and knit it in place and left the ends out so I could pull it later to reveal the stitches I needed to later pick up and knit the thumb.
There is a good video
of this technique on YouTube that was great for me. This technique can be done on any number of knitted items but, as I've noticed is usually for button holes and thumbs on mittens.