I think that what you are talking about are electrical muscle stimulators, also known as EMS. They are usually used in medical environment to stimulate muscles in those who can't use them, to avoid their atrophy. But no, these machines do not work as they are advertised. You will burn more calories by walking to the shop to buy this shit, rather than by using it. So this stuff has its application in a clinical setting, but if you are doing sports, don't expect anything reasonable from these things.