wool - n - gather

Through back loop is a common method of twisting a stitch. It can done with a knit and a purl stitch.

When making a knit stitch through the back loop, insert the right hand needle from right to left into the stitch so that the tip points left and away from you, complete the stitch as normal. When making a purl stitch through the back loop, bring the right hand needle around the stitch at the back and insert it into the stitch from left to right so that the tip points right and towards you, give the need a quarter turn so that the needle is in normal purl position pointing left and towards you, complete the stitch as normal.