One of the key concepts that a good tummy exercise machine would use is isometrics. You see the stomach muscles are just like any other muscle. If you use “progressive resistance” all you will succeed in doing is increasing the size of the abdominal muscle. In other words, you will make your stomach bigger. That may not be attractive whether your waist is big from fat deposits or from muscle, the idea is to have a lean flat stomach and a smaller waist. That applies whether you’re a man or a woman in my opinion.
Most good abdominal exercises are in some way a form of the simple crunch. Regardless of which part of your body you are raising in an exercise (your hips, your shoulders, or your legs), your goal is to contract your abdominal muscles. This is the foundation of any good abdominal movement and what gives it its power. Always keep your midsection tense throughout the entire movement. Do not hold your breath but try to breath as naturally as possible. If you want to improve your body shape, the abdominal workouts are an important part of the training.