Bruce Lee did dragon flags often, but they were popularized by Rocky when he was training to beat the hell out of Ivan Drago in Rocky IV. They're exceedingly tough to do, especially if you've never done them before. They're also pretty much the most impressive looking movement you can do in a gym.
Even though they're tough to master, this relatively easy progression takes you from reverse crunches to true dragon flags in no time.
Within the realm of training for greater strength, muscle mass, and endurance lies an area of science that remains relatively untapped: Neuroscience. It's indeed the uncharted waters in the vast ocean of the science and practice of resistance training. That's because so little is known about how the nervous system actually works.