Behind today’s title one would actually suppose an exercise for the legs, but it isn’t. It’s an exercise for the lateral abdominals.
I lay on my back with slightly bended legs and the arms crossed over my chest. I lift my upper back up a bit and start to swing the left leg to the right passing over the right leg by rotating also a bit the hip. Then I go back into the starting position and repeat the same with the right leg. I go on alternately.