This exercise will improve strength throughout your oblique abdominal muscles.
Start by lying on your back with your knees bent. Take a deep inhalation through your nose passing the air down into your lower rib cage. As you exhale fully try to pull everything up and in to engage the abdominals. Place a ball or towel roll between your knees, press with 50% tension and lift both legs to a tabletop position. Rotate your knees left to right without letting your lumbar spine lift far from the ground. Complete 15-20 times.