Whatever. I don't know it's impossible, but I believe it is. An analogy would be standing with your feet together and your upper body at 45°. Same principle - your centre of gravity is outside your base. In the standing case your base is your feet. Your centre of gravity would fall a couple of feet outside your base. With dragon flags, your centre of gravity would be somewhere around your dan tien (or a bit higher maybe). Your base is shoulders/neck. So your centre of gravity is way outside the base for most of the ROM of the exercise (except when your feet are vertical). You need the hands holding the bench to counteract the centre of gravity force. The abs are worked by keeping the legs and torso straight as a board. The exercise you're probably thinking of - leg raises with your butt on the floor - you have everything from your neck/shoulders to butt on the ground, so your centre of gravity is inside your base. Make sense?
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