Easy to do and easy to progress — just add weight. It's a small motion: no full sit-up. Bring your chest as close to your hips as you can without lifting all the way up.
To actually train your abs you need to flex your spine — made much easier by putting a bar behind you for support. Raise and curl rather than just swinging the legs.
All about flexing or bending your spine: keep the hips fixed in the same place and bring the chest closer to the hips by rounding your back on the way down. The back starts arched and ends rounded.