Everybody wants flat abs but few people know how to attain them. The best flat abs exercises have a lot to do with the individual, the type of exercise they enjoy, if they want an abs machine or will use physical exercise, and how much they weigh, or, how well toned they are.
Some ab machines are low intensity. This means that you can attach the components to your body and do other things while the machine contracts your abs. Other abs machines require you to physically move parts of the machine (think rowing types of machines) which in turn exercises your abs.
Before buying any exercise equipment it's worthwhile reading product reviews, and if possible, trying out the machine at a gym to find the one that best suits your needs.
If you are not used to doing physical exercise, it's always best to consult with your doctor first, and, if you have any medical issues then you should consult with your doctor before starting any new type of exercise program.