I don't know what you would expect from such a machine.
Assuming you are overweight through obesity and not mucle bulk seen in rugby players and so on, you will not get much benefit from these machines.
Used regularly and properly it will strengthen and tone the target muscles, but at 100lbs overweight you would never really notice any difference.
All the good work would be hidden.
I think this particular exercise would be very difficult for you at the moment.
Concentrate on losing weight by good diet and regular general exercise - brisk walking, swimming, cycling perhaps.
Also stretch properly. Good flexibility is important to everyone, and it will help with exercise and posture, making you look and feel better. Spend time learning to stretch safely and properly, and ten minutes twice a day will be a big benefit.
As the weight comes off, you can incorporate target specific exercises such as ab crunches.