Starting Position: Grasp the dumbbells in your hands and sit with your back against a back rest so that your head, shoulders and butt make contact with the bench. Place your feet firmly on the floor. Stiffen your torso by contracting your core and abdominal muscles ("bracing"). If working with a spotter, signal for assistance. Exhale and slowly lift the dumbbells to your shoulder level with a closed, pronated grip (palms facing forward and thumbs wrapped around the handles), with the dumbbells at shoulder-width or slightly wider and wrists in a neutral position (avoid flexion and extension of the wrists). Retract and depress your scapulae (pull down and back) and hold this scapulae position throughout the exercise.
Upward Phase: Exhale and press the dumbbells overhead in unison until your elbows are fully extended without any arching in your low back. Maintain your body contact with the bench, neutral wrist (avoid flexion and extension) and depressed, retracted scapulae positions. For more comfort through the range-of-motion keep your elbows pointed in front of you.
Downward Phase: Inhale and slowly lower the dumbbells in a controlled manner by flexing (bending) your elbows while maintaining a rigid torso, neutral wrists and contact with the bench and floor. Avoid simply yielding to gravity and consciously try to contract your lats and pull the dumbbells down towards you.