This exercise works the upper and lateral abs.
Lie down into supine position, your waist should be on the BOSU, legs on the ground in a straddle. Slowly lift your shoulder upwards, towards your knees so that your shoulder blades elevate from the ground 3-5 cm; in the meantime rotate your torso to the side as if you would try to touch your left knee with your right shoulder and the other way around. Hold the position for a second then let yourself go back to the starting position so that you gauge on the BOSU then repeat this. Perform the exercise to both directions alternately.