Stand with feet shoulder-width apart and perform large vertical circles with your straight arms.
While one arm if rotating forward, the other arm is rotating backward; it will probably take a few attempts to learn the coordination necessary to perform this exercise.
Perform rotations in both directions; each rotation counts as one repetition.
Aim for your arms to go all the way behind.
Form tips
Maintain a relaxed but engaged posture throughout the exercise.
Keep your core muscles lightly engaged and avoid arching your back.