Stand holding a dumbbell by your side, with your elbow very slightly bent.
You can place your free hand behind your head (as in the illustration), on your hip, or you can just keep it by your side.
Inhale as you lower the dumbbell by laterally flexing your waist until you feel a mild stretch in your side.
Exhale as you laterally flex your waist equally as much in the opposite direction to raise the dumbbell.
Repeat for the prescribed number of repetitions.
Repeat the exercise with your opposite arm.
Comments and tips
Keep the dumbbell close to you side, your elbow very slightly bent, and your hips still.
Keep the movement slow and under control.
Do not laterally flex your waist beyond feeling a mild stretch in your side.
Make sure that most of the movement occurs in your thoracic (upper) spine, which is designed for mobility. Do not keep the movement in your lumbar (lower) spine, which is designed for stability.
The dumbbell side bend is effective at targeting the internal and external obliques, strengthening the lateral flexion of your spine, improving spinal mobility, and helping to develop a strong and stable core.