By knitting in to the back of a stitch you create a twisted stitch, called knitting through the back loop (ktbl).
How are twisted stitches abbreviated? Usually the stitch is abbreviated as tbl (through back loop). You might also see it in knitting books as knit tbl, ktbl, or ptbl (knit through back loop, purl through back loop).
Twisted stitches areworked by knitting (or purling) in to the back of the stitch. This isusually most obvious when done in stockinette stitch- it is not nearly so noticeable when done in garter stitch. The stitch looks like a tiny "cross" is formed in the stitch, and the twisting pulls the yarn tighter. You can do other stitches through the back loop- for example, knit 2 together through back loop (k2tbl), which is a left leaning decrease.
Twisted stitches can be a great way to really make stitchesstand out- useful for making cables "pop", since it pulls in the edges of the stitches.
Uses for Twists Stitches
Twisted stitches are usually a variation to that is decorative- i.e. more for looks than necessity. Nevertheless, some people may knit a row of twisted stitches after picking up stitches (usually for socks after the heel) to tighten up the stitches and to keep the fabric from gapping.
Most of the time though, twisted stitches are just a nice way to make your knitting look more dynamic. By knitting through the back loops in stockinette stitch, for example, your knit fabric will have some "curve" to it- it won't want to lay nearly so flat as regular st st.
Twisted stitches are especially useful for 1x1 and small cables, because the twisted stitch pulls in the edge. It is alsogood for for making plain fabric look more interesting (I like a fabric where I knit a row of twisted stitches, then purl back in thenormal way with no twisted stitches). For ribbing, twisting all or some of the stitches (i.e. I often doonly the purls in 1x1 or just the knits) helps keep the ribbing from "puffing" out as it is occasionallyprone to doing.
Knitting patterns will always designate what stitches are twisted- with both the abbreviation, and if there is a chart in the pattern, the twisted stitches will have a different symbol than regular knit and purl stitches.