Hmmm, I agree with Lara in that I’ve never heard this question before. It absolutely works the core muscles. I prefer to do it by lifting the hands up toward the ceiling on the inhale, then raising the head and rolling up on the exhale. It’s not as hard on the neck as the neck pull which I prefer not to teach. When rolling down it is important, as Jill says, to pull the belly button to the back as you initiate the move. Once on the mat, I let them slowly roll their heads from side to side to release any tension on the neck.
I started Pilates at 250 pounds and could not even to one situp. I credit roll ups and roll downs as the one exercise that really increased my core strength enabling me to do situps with ease.