its one of those things, you can't see how it could realistically work and it doesn't sound like anything but a scam.
but your muscles do contract, maybe not much, maybe not under any load, but contracting muscles does take energy doesn't it? I can never decide. I don't think its gonna do much, or help particularly. I mean, if you sit at your desk and do tighten your abs all day you will burn more energy than if you sat there and didn't do it. The electrical impulse is "easier" and can be done without any concentration, without the boredom and while focusing more clearly on something else.
I can't imagine it burns no calories. But then exercise burns very little energy, much less than you'd expect at full intensity straining all your main muscle groups, so one group barely strained is going to do even less. will you be straining the muscles hard enough to tear muscle fibre's to encourage new growth, highly unlikely, will it burn some calories, probably. if you're losing weight over the period of a year and have a long way to go will it add up, probably, is it worth it? still probably not.
But its another tool, it can help you feel your doing something, help you feel a diet is working, help you mentally stay on. Maybe having a really bad day, can't get to the gym and you feel down, it gives you something that you can say is helping you. meh.