This video knitting tutorial will help you learn how to knit the purl through the back loop stitch. This technique is used to twist stitches on the purl side of the work. It can be a little awkward for beginners, but it is easier than it might seem.
Step 1: Insert your working needle into the back of the next stitch on your main needle, knitwise (same direction as the main needle). Then swing the working needle around under the main needle so it is facing purlwise.
Step 2: Wrap the working yarn around the needle as if to purl and work the stitch as normal.
Step 3: Pull the stitch off the main needle.