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Right off the bat, Elvgren is giving us cues to motion that aren't present in the photograph. We see motion blur on her buttocks (second in line as focal, after her face) and the machine (arguably third). Her hair is flung out in exciting directions. She's even been given a dangly earring that sways, a subtle but effective touch (also note the addition of matching bracelets on her right hand). In reaction to this more violent jostling, he's spread the girl's legs apart, stabilizing her.
Elvgren's model, it's safe to say, wasn't posing with an actual exercise belt. This was obviously some jury-rigged, elastic proxy, for which Elvgren later aquired reference. Since it doesn't even appear to be supporting her weight, as a real machine would, Elvgren has made sure to tilt her torso back in the saddle.
The photo was taken in a studio, so all ancillary background elements have been removed in the painting. The lighter backdrop afforded him the opportunity to lose edges. And you can see all tangencies have been neatly eliminated.
There's a general streamlining of form that's taken place. Any blockiness or stiffness of limbs has been softened. The pinched-in areas, not present in the photo, show her skin to be yielding.
Bringing her torso back does 3 things :
-creates a squash just above the buttocks
-brings the arms back, obscuring the breast (which, in this instance, is not where Elvgren wants us to look)
-allows the head raise and tilt
Elvgren rightly chose to make the waist band lighter, so as not to blend into her garters, and to help further round the buttocks in his rendering.
Note the very readable graphic "stamp" of the painting vs. the muddier, incomplete stamp of the photo. The prop is used as a framing element.
