By knitting in to the back of a stitch you create a twisted stitch, called knitting through the back loop (ktbl).
Abbreviation: tbl (through back loop). You may also find it in knit patterns as knit tbl, ktbl, or ptbl (knit through back loop, purl through back loop).
Twisted stitches aremade by knitting (or purling) in to the back of the stitch. This isoften most apparent when done in st st - 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 also do other stitches through the back loop- for example, knit 2 together through back loop (k2tbl), which is a left slanting decrease.
Twisted stitches can be a great way to really make stitchescatch the eye- useful for making cables "pop", since it pulls in the edges of the stitches.
Why do twisted stitches?
Twisted stitches are usually a variation to that is decorative- i.e. more for looks than necessity. On the other hand, many people may knit a row of twisted stitches after picking up stitches (often after finishing the heel of a sock) to tighten up the stitches and prevent holes.
Most of the time though, twisted stitches are just a nice way to make your knitting look more dynamic. If you knit through the back loops in st st, for instance, your fabric will have some "twist" to it- it won't want to lay quite so flat as normal stotckinette stitch.
Twisted stitches are especially of use for one by one and small cables, since the twisted stitch pulls in the edge. It is alsouseful for making normal knitting look more fascinating (I like a fabric where I knit a round of twisted stitches, then purl back in thecommon way with no twisted stitches). For ribbing, twisting all or some of the stitches (i.e. I often donothing but the purls in 1x1 or just the knits) helps keep the ribbing from "puffing" out as it is at timesdisposed to doing.
knit patterns will always note 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 typical knit and purl stitches.