Kim Kardashian has shared a new Instagram shot of herself, and she's all corseted up.

"I'm really obsessed with waist training!" she captioned the selfie, which shows off her accentuated hourglass figure.

Her sister, Khloe, shared a profile photo of herself six months ago, saying she, too, was "obsessed."

And their friend, model Blac Chyna, Instagrammed last week that she's "obsessed."

They're embracing a trend that has transformed the olden day Victorian corset into a modern day "waist trainer."

Or maybe they're all just helping out their friend, Premadonna, a Miami entrepreneur who sells the products at Whatsawaist.com.

So why the obsession with what is essentially a harsher, retro version of Spanx?

"The products are easy to put on, relatively comfortable, affordable, and provide an instantly slimmer midsection. It's a quick and easy thing to do," says Ruben Soto, president of Hourglass Angel, a company that offers waist cinchers and waist trainers.

He says that sales of his top three waist trainers are up 250% in the last year. His best-selling product? The "Amia" waist cincher. Price: $54. It promises: "Wear it for an instant 1-4 inch reduction in your waistline."

Jessica Alba wore a similar waist band after her pregnancy in 2009, but she called hers a "girdle."

Brooke Burke-Charvet, mom of four and former Dancing With the Stars co-host, invented her own version of a waist slimmer -- Tauts, targeted at post-baby belly "shrinking." She explains on her website that it's an "age-old secret," and that her French mother-in-law introduced her to the concept. Burke-Charvet then created Baboosh Body, a product made out of nylon and neoprene and designed to be used during workouts. "It's going to promote intense core sweating. Wrap it around, as close as you can get it, next to your skin," she says.

Old-fashioned corsets, says Soto, had laces on the back that were tightened to the point of actually altering the ribs to create the smaller waist line. In other words, he says, they worked by "reshaping the lower ribs." "Waist trainers," he says, "use compression as well — to a lesser degree — but are much more comfortable."

And do they really work?

It's nonsense, scoffs Stephen Ball, associate professor of Nutrition and Exercise Physiology at the University of Missouri in Columbia.

"Quick fixes are what Americans want, and, unfortunately, they typically are what I call quackery," says Ball. "You aren't going to lose body fat by cinching the waist, you're going to lose by exercise and watching what you eat."

Ball, speaking while working out on his treadmill desk in his office, says, "There is no physiological reason why (a waist cincher) should work," he says. "It's a perfect example of people being more worried about appearance than good health."

Soto acknowledges that there aren't scientific studies to confirm that the waist trainers will make you lose inches. It's all about sweating, he says. "This effect can be compared to sauna suits used by athletes for weight cutting, however, it's done to a smaller degree with waist trainers."

Richard Cotton, exercise physiologist and director of certification with the American College of Sports Medicine in Indianapolis, argues that waist cinchers "work" depending on how you define efficacy. "They work to provide you with a slimmer look, but they don't do anything to actually reduce body fat," he says.

They could provide support for the core, but if you wear them all the time, they could decrease core strength because of that support. "And that's not really a good thing, because of lack of muscular stimulation, so they could be harmful in that manner," says Cotton.

Even Soto admits, "We do not have evidence that it has a permanent effect after discontinued use."

But they definitely help give you an hourglass figure -- if that's what you're looking for.

Contributing: Maria Puente