Called shirring. However, the appear it generates is similar to smocking, which can be an embroidery strategy that’s been around a long while. But probably don’t google “what on earth is smocking” cuz apparently What's more, it refers to something which has very little to try and do with sewing… https://shanenyglr.qodsblog.com/31790782/helping-the-others-realize-the-advantages-of-covered-spandex-yarn