I read a Frederic Delavier’s book « Strength Training Anatomy » and I learned good stuff.
Lying on your back with your hands behind your head. Your thighs vertical with your knees bent :
Inhale and lift your shoulders off the floor by bringing your knees closer to your head by rounding your back.
Exhale at the end of the movement
This exercise works mainly rectus abdominis
To work more intensively your obliques, you can bring alternately your right elbow to your left knee and your left elbow to your right knee.
Share this article if you think it can help someone you know. Thank you.