I see two slightly distinct ways to do kneeling ab wheel rollouts.
Highly similiar to the first in the outward part of the movement, but the guy actually advises to slightly crunch in the inward part. As seen here.
My observation is that when extending too far, I cannot do the backward portion without a slight, active tuck from the abs. For me, as I have lower back problems, I will do the first version and probably use a door or something to limit range of movement to something managable. Better safe than sorry and all that.
Ultimately, which version is safer, which is more effective and why?