Stand with the feet around shoulder width and hold a barbell on your back like you would for a back squat.
 
Keeping your entire trunk and glutes tight and your hips and shoulders squared off facing forward, bend the trunk to one side as far as possible and then to the other with a controlled tempo.
 
You can emphasize the whole, lower or upper trunk by limiting the motion to each segment. For example, keeping the lower trunk more rigid and focusing the motion on the thoracic spine.
 
Purpose
Lateral trunk flexion is usually neglected in favor of forward flexion, but the sides of the trunk provide an important stabilizing function for the trunk and spine. The barbell side bend is a very simple but effective method of strengthening the lateral trunk and makes it easier than a regular side bend to restrict motion to specific trunk segments.
 
Programming
The barbell side bend can be done for sets of 8-15 reps/side.