Inverted Stomach/Bulging Hips??

Does anybody know why certain women have the ability to invert their abs behind their hips ? This is very hard to explain. My wife does this exact same thing just by simply leaning back but she doesnt even have abs. I think it might just be overdeveloped obliques ?

Very nice look to be honest. Does anybody know what its called, what muscles it is, what causes it or simply just wants to share their findings ?

few examples

Those are developed oblique muscles. They control bending and twisting the waist.