Hanging Leg Raise. This is an advanced lower abdominal exercise that is frequently performed incorrectly. This exercise also requires additional equipment, such as a sturdy bar that you can hang from. If you want to maximize lower abdominal activation, follow these tips:
This exercise requires a significant amount of upper-body strength, because the hands and arms must support your bodyweight.
Grab onto a bar above your head and hang as still as possible. Then bring your legs up until your thighs are about parallel with the ground and your knees are bent about 90 degrees. This is the starting position for the exercise.
Contract your lower abdominals and pull your legs closer to your chest, without excessive rounding of the lower back. This exercise has a small range of motion, but if done correctly, is very challenging for the lower abdominals.
Return your legs to the starting position. You may let your legs go somewhat lower than the starting position if you are able to maintain the contraction in your abdominals and prevent your lower back from arching. The lower back should stay flat just as in the floor exercises.
Do not swing or use momentum to help raise your legs. Swinging is the most common sign that you may be cheating or using muscles other than the abdominals. This should be a very controlled movement and you should focus on feeling the lower abdominals pulling the legs up instead of trying to jerk your legs up.
Do not straighten your legs toward the ground between repetitions — keep your knees bent. Straightening your legs (perpendicular to the floor) changes the exercise so the emphasis is on the hip flexors instead of the lower abdominals. Dating blogs
To increase difficulty, perform the exercise with straight legs (starting with your whole leg parallel to the floor) or add ankle weights.

It is a really good exercise and you should definately feel your lower abs the next morning because they should be sore. I usually do 3 sets of 6 or 8. If it doesn't do anything for you… combine other ab exercises with this one to apply double the pressure.