Knitting through the back leg (often referred to as the back loop) of your stitch, creates a twisted knit stitch, with the twist to the left.
Background On Front and Back Leg
When a knit stitch is created, it has the a leg (loop) that sits in front of the needle and a leg that sits behind the needle. The leg in front has the yarn coming from the stitch before it and the yarn transitions over the top so the back leg has the yarn either going to the next stitch or the yarn.
To knit a straight stitch, you knit through the front leg and the route of the yarn through the stitch being made continues and matches the path above. To twist that knit stitch, knit into the back leg and the two legs of the previous stitch are crossed at the bottom as the new legs are established (the new legs are not yet twisted).
Working Flat and Purling. If you tension your yarn in your left hand (generally any of the non-throwing methods) and work your purl stitch so that the yarn goes directly under the right needle and is drawn backward onto the right needle to create a new stitch, the new stitch will be a twisted stitch. On the next row, those will be knit stitches, which need to be knit through the back leg to untwist them (this is a bona fide method of knitting called Eastern Uncrossed.). If the stitch is not untwisted on the next row, it will be a twisted stitch, but the twist will be toward the right. This is desirable for some types of knitting, but is not what most knitting patterns assume the orientation of your stitches will be.
Knitting through the back leg (loop)
Abbreviation: k1-b in Jill Wolcott Knits® patterns or k1-tbl
Knit 1 through the back leg of the stitch (the needle goes into back leg without going in front of left needle).
Purling through the back leg (loop)
Abbreviation: p1-b in Jill Wolcott Knits® patterns or p1-tbl
Hold yarn to purl, but bring needle around back of left needle and go into back loop from left to right, then purl.