Once you let your body go concave (the opposite bend of waist that you are trying to avoid) your putting load on low back and wont be able to get back up. Negatives are ok, personally i prefer the on floor rather than bench with arms on a ladder or a bar above the head so your much more vertical. Just go down as far as you can to go back up. I look at dragon flags as more of a test than an exercise. Would rather the bulk of training come from heavy leg lifts or crunches. If you can knock out straight leg lifts on slant bench w a 30-50lb dumbell your abs are strong imo.
That'll work my hip flexors primarily and abs secondarily. I want to do the opposite of possible. It's one reason why i loved the ab pulldowns so much... it's the only time i've ever gotten an ab pump in my life. A freaking pump. im my ABS. not a burn! a PUMP. i sat down to take off my shoes after the workout and my stomach was in the way!!!
i also liked them because i could increase resistance on them consistently until the weight stack is maxed out, or i started lifting off the ground (at which point i would need to hook my ankles under something).
if only the ab pulldown went up to 200lbs and was facing another machine that i could hook my ankles under...