New optical-illusion dresses by Stella McCartney, Hervé Léger, Victoria Beckham, and others make curves more dangerous—even as they slim. That women will stop at nothing to artificially achieve the perfect silhouette is not breaking news. How they do it, however, is ever-changing. In the past, undergarments did the bulk of the work, from whale-bone corsets to Real Housewives–designed faux Spanx; women’s waists have been nipped, tucked, and cinched underneath their clothes in every which way. But now, thanks to a slew of optical-illusion dresses hitting red carpets and runways, the dresses are drawing the curves.