Hello all!

I’m very familiar that the standard DIY technique of shirring in a normal sewing machine (the top needle uses standard thread and the bobbin uses Elastic thread) but all my  tops brought from shops/brands are MUCH better and are made using elastic on both sides of the fabric. (See photo)! does anyone know how on earth this is done???? Looks like a chain stitch of some kind in the back.. is this in a different machine maybe?

thanks
nicole