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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.
Weightlifting belts typically aid a lifter by helping to brace the core, which typically is thought of to be the lower back, abs and all the stabilizer muscles in the midsection. When one lifts maximal weights, it's typically the core that goes first or acts as the bottleneck for being able to lift more weight.