This is a fantastic workout for someone who spends a lot of time sitting at a computer. It's also beneficial for improving your posture.
To help you maximize the rotation, make sure your thumbs are pointed outwards. Make sure your elbows are tucked into your body and your chest is elevated as well.
This exercise can be done with both shoulders at the same time. However, we suggest beginning with only one shoulder at a time so you can focus on the movement and ensure that neither arm is working harder than the other.