In the case of the picture I showed, she just took a length of fabric, sewed it on, then whacked it off. If you measure across the middle, cut, then sew, you won't end up with the waves. As your feed dogs grab the fabric, they can slide the lower fabric more than the top, if they are not held securely, as what happened here. This is how you get ruffles.
When you are making garments and want to ease in fabric, you put the larger under the smaller to ease it in.