Hattori recommended this move to work your shoulders and arms. Key tip: keep your core tight and avoid arching your back.
Start standing with your feet shoulders-width apart and a slight bend in your knees. Hold the dumbbells down at your sides, palms facing towards each other.
Turn your hands so your palms face up. Bring the weights to your shoulders, performing a bicep curl while keeping your core tight and your back flat (not arched).
With the dumbbells held at shoulder-level, turn your palms to face outward.
Straighten your arms to press the dumbbells overhead, extending until your arms are straight (but don't lock your elbows).
Bend your elbows, lowering the dumbbells back to your shoulders. Turn your hands so your palms face your body as you lower them.
Lower the dumbbells to your sides, completing the bicep curl movement.
Turn your palms towards each other again as you come back to the starting position.