Also be sure when you are moving your arms backwards you are turning your body completely to the 'hug' position, not turning completely causes your shoulder to turn/twist in a bad way. I had a big problem with shoulder pain until I watched myself on a webcam recording. Recording yourself just to look for bad posture is very helpful because so many times I kept falling back to bad habits.
Beginners do it until they can get it right, experts do it until they can't get it wrong.
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