I love this exercise. It's a great rotational ab exercise that powerlifters and athletes alike should use.

Hold the collar end of the bar with both hands. Keep your elbows at your side. This will shorten the lever, making the exrcise a bit easier. Turn your shoulders and hips to one side. Using your obliques and hips drive the bar back to the middle and repeat on the other side. Alternate sides for 6-12 reps each side.