Pull the knees, crush the lower abs.
Drive the movement from your abs, not by yanking with your hip flexors.
The leg pull-in works the abs from the bottom up, the core curls the pelvis toward the ribs as the knees drive in, while holding the legs off the floor between reps keeps the tension constant from start to finish. The lean-back position turns your torso into a counterweight, so the abs never get a moment off. It earns its place as a lower-ab focused move that needs nothing but a bench edge or a patch of floor.
The hip flexors always join in when the legs move, but they should not run the show. Curl your pelvis under as the knees come in, think of tucking your tailbone, and exhale hard at the top to force the abs to lead.
The bench lets your legs extend below your hips, giving a longer range and a bigger lever, so most people find it harder. The floor version is the friendlier starting point, then graduate to the bench edge.
Slow enough that your feet never touch down and your torso never rocks for momentum. A controlled two-count out and a deliberate pull in beats fast piston reps every time, especially on the extension where the abs resist the legs straightening.
