Dragon flags are an amazing exercise to improve your relative strength. It hits your entire anterior chain, making it a great value-for-time exercise.
Try to keep your body as straight as possible throughout the concentric and eccentric phase of each rep. However, this is very difficult and requires a lot of requisite strength. If needed, modify to a bent-knee dragon flag on the concentric phase (as seen at the end of the video) and then straighten your leg(s) on the eccentric phase to maximize strength and build the requisite strength to maintain a straight body throughout the movement.