Lie flat on the floor with your lower back pressed to the ground (pull your navel in to also target your deep abs).
Put your hands behind your head, then bring your knees in toward your chest and lift your shoulder blades off the ground, but be sure not to pull on your neck.
Straighten your right leg out to about a 45-degree angle to the ground while turning your upper body to the left, bringing your right elbow toward the left knee. Make sure your ribcage is moving and not just your elbows.
Now switch sides and do the same motion on the other side to complete one rep.