One thing I used to remember when growing up is a device that Bruce Lee used to use to stimulate his muscles. I would see him use this device and see his muscles contract and relax automatically.
Twenty something odd years later I see the same device used to tighten and firm up your abdominal. They all have various names like the Flex Belt see review here, however they’re all marketed differently.
Essentially though, they’re all the same kind of contraption. They electronically stimulate the muscle to contract and relax without you even thinking about it.