It seems to me that many Golf Stroke Engineers stress that the swing only goes up, back, and....then down, out, and forward. Three dimensional. Also, forward is a result of two divergent forces (down and out).
1)Regarding the arms: If an arm is straight, is this as far 'out' as the arm can go during the swing ?
2)From the top of the backswing, where is the aiming point in relation to being down, out and forward ?
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"Golf is not a subject but a motor skill which can only be learned and not taught." - Michael Hebron
"The Body, Arms and Hands have specific assignments during the Golf Stroke, and they must be coordinated into one efficient motion." - Lynn Blake