Have your arms hanging off your shoulder sockets by the weight of your arms. This will give you the "box on the neck" upper back look you want, as opposed to a dead straight upper back.
Another good check to do is from the 'down the line' view (golfer between camera and flag). You should be able to see your left arm. This means your right shoulder is in a nice low position (relative to the left), and your right elbow isn't too high, but relaxed.
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