Here's a good piece of info in regard to leg raises, about the mechanics of the movement:
And from the exrx.net entry for Lying Leg Raise:
" Rectus Abdominis and Obliques only contract dynamically if actual waist flexion occurs. With no waist flexion, Rectus Abdominis and External Oblique will only act to stabilize the pelvis and waist during hip flexion. It may be necessary to completely flex the hips before waist flexion is possible. "