NEMS work to stimulate muscles that are weak and that are not able to attain a full contraction without additional stimulus. Once you can achieve a full active muscle contracttion, as @Ivo says, you want to actively exercise the muscle and add resistance to achieve further strengthening.
The NEMS does produce a muscle contraction, so in that respect, your abdominal muscles are "working" when you wear a unit. If you actively contract along with it, you will get a better contraction. It can remind you to contract your abs. But according to the FDA
While an EMS device may be able to temporarily strengthen, tone or firm a muscle, no EMS devices have been cleared at this time for weight loss, girth reduction, or for obtaining "rock hard" abs.
Also, electrical stimulation units can also be contraindicated for people with pacemakers, metal implants or medical conditions such as cancer.
If your primary goal is to lose weight and reduce your waist circumference, focus on your diet, exercise (cardio with HIIT and resistance training), and avoid prolonged sitting - get up and move frequently. Track your percentage of body fat in addition to your waist circumference to assess your progress.
The following q/a's should help you with your goals to start losing weight and reducing your waist circumference:
- Best exercise to lose belly fat fast
- How can an overweight engineer get back in shape?
- Optimal exercises for an abdominal workout