Oh yeah, I know what you mean. To be honest, I’m not completely sure how exactly it’s supposed to work, but I’ve just tried to stick to the principle of maintaining a braced and neutral a position as is possible. In this regard, I guess I would say don’t think of it as keeping your back flat, but rather as neutral as you can, and keeping your pelvis as even, and as close to the ground as you can, while doing the stretches. Don’t push it further than you can without breaking into a faulty position. Not being able to achieve the full stretch in a stable position is likely just another indication of mobility restrictions that need to be worked on. Maybe do this hip opener exercise, and test to see if it improves your ability to keep your lumbar neutral while in flexion?