As with a lot of members on the list, I've been have a hell of a time trying to complete straddle-ups (MN/PE2) with the full ROM from the video. I'm not there yet, actually not all that close, but since reading about suggestions on how to slowly improve, I've implemented some of these with some success. Sitting in a straddle and raising my legs while my upper body stays upright seems to be helping with my hip flexor strength. I've also been straddling against a wall and sitting up while simultaneously pulling my legs toward my torso to accentuate the compression aspect.
But most of all, I've really been working hard on the Jefferson curls. While doing them, I've been intensely focusing on the "focus points," uncurling/curling my vertebre one at a time and really concentrating on keeping my hips tucked. This has greatly improved my hamstring flexibility. For the first time in my LIFE I can fully touch my palms to the floor. Slowly but surely.... I'm getting there....