If you have ever seen the abbreviations ktbl and ptbl (or k tbl and p tbl) in a knitting pattern and wondered what to do, it simply means to knit or purl through the back loop/leg of the stitch instead of the front loop/leg. Working in the back loop of a stitch essential twists the stitch.
Think about this. Whenever you knit or purl a stitch through the front loop/leg, the stitch you are working into rotates counter-clockwise when it is slipped off the left needle. This is the way most knitters form their stitches most of the time.
When you knit or purl through the back loop/leg, the stitch you are working into rotates clockwise when it is slipped off the left needle.
Pattern: Slipstream Cardigan
Yarn: HiKoo Sueño
Needles: Brittany Birch Single Pointed Needles