Hopefully it is obvious that the one on the right, where the man has a straight back as opposed to a curved one, is much better. His head, neck and back are nicely aligned which gives him a mechanical advantage over the other chap.
Actually there are many more subtle considerations which aren't so easy to see. Ideally, the relationship between the head, neck and back should be poised, not fixed; there should be no narrowing across the front of the chest and there should be a sense of elastic distance from joint to joint in the arms and legs.
This position is one we study ad infinitum in the Alexander technique. We call it 'monkey' and there's no better application of it than on the golf course.
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