Stand with your back straight, feet hips width apart and knees slightly bent.
Reach one arm forwards and the other back to shoulder height.
Keep your hips and lower body square by activating your core stability muscles, but allow your upper body and shoulders to rotate with the arms.
Swing your arms to change positions, rotating your upper body with the movement.
Increase the speed at which your swing your arms, making sure you hips and lower trunk remain square.