The blur and doubling effect along the side of her torso and waistline is not a double exposure, motion blur, or Photoshop effect.
As best I can tell, it's the shadows from both the metal grate and her arm being cast onto her body, and then the light from the other side of her being reflected from her arm back into those shadows.
When I first saw how this pose was shooting, I thought my camera was malfunctioning or my eyes were playing a trick on me. But it's not the eyes, it's the light and shadows, playing their tricks to unexpected, and I think beautiful, effect.