This exercise is a great abdominal exercise to work on obliques (the side of your abs) and rectus abs (the long flat muscles along the front sides of the abdomen - 6-pack portion). You fully extend out one leg while the other is bent and you touch your opposite elbow to the bent leg as if you are doing a twisting crunch. Then you repeat for the other side without taking any rest or dropping your torso back down so the whole time you are doing them your upper body is in a crunch position. It's tough. Use as big a range of movement as possible until you fatigue then you can make the movement smaller.