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Ab belts may cause muscle contractions if used properly. But the belt alone cannot produce six-pack abs. The real challenge to ab definition is the layer of fat that lies right under the skin, hiding muscle tone.
For those of you wondering about the physical properties of the belt: it's basically just an elastic loop that goes around your hips, with small clips dangling in the front and back. You have to get special pads to fit these clips, with loops that either hook onto the clips, or ones that you secure with safety pins.