Lie on the ground, facing up, with your hands behind your head.
Use a Hollow body position: engage your Abs and lift your upper back and legs off the ground. Only your lower back should be in contact with the ground at this point. This is the starting position.
Bring one leg in and crunch forward to touch that knee with the opposite elbow.
Return to the starting position ( Hollow body ) and repeat on the other side.
Form tips
During the duration of the exercise, only your lower back should be in contact with the ground.