Usually, stitches are worked from the front loop, but it is also possible to work them from the back. This twist the stitch.
The stitches worked through the back loop are thicker and tighter than classical stitches (worked through the front loop).
In this tutorial, you will learn how to purl stitches through the bacl loop (Ptbl)
Notice the front and back loop of the next stitch on the left needle. Your yarn is on the front of your project.
Put the point of your right needle through the back loop of the next stitch on the left needle, from the left to the right.
Wrap the yarn around the point of the right needle and pull it through the loop. Let the stitch drop from the left needle. You have worked 1 stitch through the back loop.