This exercise is used to strengthen the oblique muscles and the lower back. The twisting motion is useful in many athletic applications.
Begin by sitting on the floor and placing your feet under a heavy object; as if you were doing sit ups. Or you can have a spotter hold your feet down. Next, bend your legs so your knee creates a 45 degree angle. Raise your torso so that you make a v-shape with your torso and thighs; extend your arms out in front of you straight out from your chest level. Now, twist your torso as far as you can to the right side and then hold that contraction; do not move your arms, only twist as far as your torso will allow. Next, return to the starting position. Then, twist to the left side as far as you can and hold that contraction slightly; this should be done exactly as it was on the other side. Repeat this motion until you have completed your set.
To make this movement more advanced you can hold a weight in your arms, but make sure they stay extended.