Adjust pulley approximately height of neck or head when seated. Attach belt on low pulley cable by attaching carabiner on belt buckle. Sit on bench with pulley to one side and feet apart. Place head in belt loop. With belt above ears, clamp belt snugly around head by tightly gripping ends close to side of head. Pull torso upright by bracing opposite hand on far side of bench.
Pull head away from pulley cable by laterally flexing neck. Tilt head back toward cable pulley and repeat. Sit on bench the opposite direction and continue with other side.
Sit at distance away from pulley so weight does not bottom out at lowest range of motion. Keep clinched grip on belt against side of head throughout movement. Woven cotton or cloth belts tend to slip less, particularly on bald heads. If belt slips from head, place small towel between belt and head.
Cable carabiner slips to one side on square buckles and stresses belts structural integrity under heavy loads. Rounded buckle is more ideal since it provides more even stress on belt. Belts not made specifically for weight training may cause injury if they fail under load in which they were not designed.
Exercise can also be performed off of rack with belt, nylon rope, and plate holder from wrist roller. Nylon rope offers less friction on smooth rounded fixed surface. Alternatively, non-nylon ropes can be used off of revolving barbell sleeve on smith machine. See Cable Neck Lateral Flexion with belt and rope.
See all muscle used in Lateral Flexion:
A degree of Thoracic / Lumbar Spine lateral flexion is typical involved in this movement. See animation of spinal lateral flexion during neck lateral flexion.