Rows 21-23: Knit 9 stitches on needle.

Bind off all 9 stitches; cut yarn.

Rejoin yarn where you started binding off 21 stitches. Cast on 12 stitches using cable cast-on method.
Row 21: Knit across 12 new stitches and 9 from holder. (21 stitches total) help?

Are you asking how to rejoin the yarn? Just start in knitting with it, leaving a long tail to weave in later. Don't worry about the loose tension for the moment; when you return to that spot on the next row, pull on the tail until that first stitch looks like the rest.

When I am starting a new strand of yarn like this what I do is leave a long tail -- 4-6 inches -- and start working my stitches with the new yarn. For the first couple of stitches, you want to be careful of your tension and not pull too hard on the working yarn. You also want to be careful on when you work these stitches on your first return row. When you are finished, adjust the tension on your first couple of stitches by gently pulling on the tail and then weave the tail of your yarn into the wrong side of your work.

Is your question how or why? if you want to know how,the directions above are great if you want to know why, you are creating a hole with more knitting on the other side. I have no idea what the pattern is, or what you are creating but does this make sense with what you are doing?