This exercise is called Kayak Abs because the movement you'll is similar to what kayak paddlers do. Actually people that ride the kayak are doing this exact exercise with riding it. To do this exercise you have to have good balance because you'll have to balance your whole body just with your pelvis and a very tiny part of your spine. Once you are on that position you'll have to rotate your body from side to side, moving your hips, touching with your hands on the floor on both sides of your body.
Details: Try to find something comfortable to put under your pelvis, without taking the balance you need to do this exercise, and do as much repetitions you can. You must always touch the floor in order to keep the quality of the exercise and in order to make more muscle.