Knitting through the back loop (or back leg) of your stitch, creates a twisted knit stitch. To knit a straight stitch, you knit through the front leg. To knit a twisted stitch, you knit through the back leg.
Knitting through the back loop
Abbreviation: k1-b or k1-tbl
Knit 1 through back leg (the needle goes into back leg without going in front of left needle) of stitch.
Purling through the back loop
Abbreviation: p1-b 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.