This is a combination of I-Cord cast on and bind off such that the fabric appears uninterrupted except for a horizontal I-Cord.

At the beginning of a RS row, cast on 3 stitches.
*Kfb, k1, k2tog, slip last 3 sts back onto LH needle, repeat from * to end of row.

In the pictures below, the I-Cord is knit in a contrasting colour. There is no reason why you can’t use the same yarn throughout, this was just for demonstration purposes.

You will have added 3 or 4 extra sts (depending when you stopped). On the next row, slip the extra sts over the following st to get back to the correct number of stitches for your pattern.

The sts on the needle after making the I-Cord tend to be loose. You can tighten them up by twisting the stitches as you work them.