It's not a matter of stretching the garment to get it on - it's not as if I make bias cut garments with negative ease, after all! They are not meant to be worn like stretch knits. No - it's allowing the fabric to drop and flex along the seam the same amount as it does in the middle of the panel so that the whole thing - seams included - has the same properties. If the seams are too rigidly fixed, the fabric will drop more in use than the seams, leading to puckered seams and a droopy, uneven hemline. This is a technique I was taught by a woman who worked both for couture houses in London and Paris, as well as in the manufactured garment industry, before becoming a teacher.