This pattern is made using the technique called Bosnian crochet. We crochet the slip stitches under back loop only. In this case we don't make the turning chains, but the first stitch of row always crochet under both loops in the first stitch from hook. This crochet pattern is dense, and it almost doesn't stretch by width, but it stretches by height very well.
1 row: 1 slip stitch in both loops, * 1 slip stitch under back loop *.
Rapport of pattern by height is one row.