I had never heard that those aids can be bad for you. I don’t use them because they’re an overpriced hassle, at least for me. I always feel like I’m lying about my body. As for just sucking in your stomach with your own muscles and no aid from external devices, check out the first image in this set I made. It can actually be a good way to work those abdominal muscles! Just remember that keeping any muscle tense for too long will cause problems. You have to think of it as reps, not one continuous, indefinite hold.