This is a variation of the traditional side bend that can really be difficult.
Start with a bar on your back. Offset the bar so that it is uneven to one side. Bend to the side with the longer part of the bar. Using your obliques, pull yourself back to the top until you are straight. Repeat for desired number of reps (8-12) then repeat on the other side, shifting the bar so the long side is on the other side of your body.
Start with the bar, then increase weight as necessary.