Knitting (or purling) two stitches together through the back loops is a decrease that slants the stitches to the left on the knit side of the work. It is abbreviated as k2tog tbl (or p2tog tbl).
With the right needle behind the left needle, insert the right needle through the back loops of the next two stitches on the left needle. Knit these two stitches together.
With the right needle behind the left needle, insert the right needle into the back loop of the second stitch, and then into the back loop of the first stitch on the left needle, which twists the two stitches. Purl these two stitches together.