Keep your left knee INSIDE of the outside of your left foot.
How?
Try to Throw "the drunk off of your back".
???
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But....Here is the 2004 New Year's Day version...
(Oh Yeah, edZ...another Brian Manzella ORIGINAL)
After you get to "SIT DOWN" on the downswing, feel like a drunk man is DRAPED over your left shoulder.
Use the ground to assist you in GETTING HIM THROWN OFF OF YOU both backward and upward and rearward.
Your legs will work correctly/your left shoulder will go UP AND BACK correctly/and you won't slide that knee out anymore.