Bind off English rib
When you bind off English rib, keep working in pattern to make edge elastic. Bind off with K over K and P over P (first and last stitch in video are edge sts that we bind off as K sts). The stitch that should be slipped according to pattern is Purled, next stitch is K tog with yo, then bind off by slipping 1st st over second on right needle, continue like this according to video.
To see a pattern with this technique, see: DROPS 133-12 or one of the picture to the left.