Usually people get Leg Raises and Reverse Crunch mixed up. One works the hip flexors and one works the rectus abdominis. As you raise your legs in a hanging leg raise, the first 90 degrees of movement come primarily from the Iliopsoas, Rectus femoris, Tensor Fascie Lata. It is not until the thighs hit the 90 degree mark that, the ABS (Rectus Abdominis) really fires strongly
--- Leg Raises ---
(Hip Flexion - Hip Flexors)
If you perform leg raises as most people do (the legs lift as the torso remains fixed), you are working your hip flexors and providing only isometric tension on the abdominals.
--- Reverse Crunch ---
(Trunk Flexion - Rectus Abdominis)
Your rectus abdominis has about 72 degrees of movement and if your torso stays fixed, then that means your not working it to it's full capacity.