Practice your backswing without letting your left shoulder-blade rise, i.e. if you were standing upright and you marked your shoulder-blade with North, South, East and West, your shoulder-blade would never move toward the North...
This flattens your arm swing automatically..
Do it horizontally while standing upright, then gradually bend to your setup tilt and maintain the same action to get the correct feeling..
You mean right shoulder, correct?
Always have your shoulders un HUNCHED?
Correct?
Left shoulder for a right hander....
All the left shoulder-blade should do is adduct or move to the West (away from the backbone). Stopping the shoulder-blade lifting places a restriction on the amount you can raise your hands.
If you hold your left arm out straight in front of you, with your shoulders relaxed and DOWN and then try and raise your hand higher (without bending the elbow) using only the SOCKET joint of the shoulder, you will find you can only lift your hand maybe 18 inches or so.
To raise you hand any higher you will find you have to let the left scapula slide upwards (go North)...i.e hunch your shoulder upwards.
Well, funnily enough, cyner is not a right hander...
Which shoulder, and was it up, down or around?......... but I did have a hunch........