The shift from the posterior shelf to broad hips has probably been subtle, for both have been functionally responsive to the same phenomena. Movements of the hip and butt area have had a considerable role in human erotica - because they are stylized versions of the copulatory thrust. The Polynesians must be the epitome of hip-wigglers, for they have developed it into a fine art in dances and in lovemaking. Many societies around the world use it in some form or other in erotic dances. High heels, as Desmond Morris pointed out , exaggerate movements of the buttocks, because women dramatize their sensuality with their gait.
Gestures also play an important part in copulatory attractants in other species.
My point here is that bumps and grinds call attention visually to the main zone of attraction, thereby strengthening the signal.