I use geometry to nail my setup, establishing a triangle with my arms and another with my legs. The second triangle is critical-my goal is to get my knees in line with my feet and hips (left photo, above). Getting this right makes managing weight shift easy. Once I'm set, I focus on maintaining both triangles as I start the club back. The trick is to swing my arms and turn my chest at the same time. I've done it right if the buttons on my shirt continue to split the middle of my top triangle (right photo). My lower triangle? I leave it alone.
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I can rotate my shoulders past 90 degrees without thinking, which sometimes makes it difficult to get everything back to where it needs to be at impact. More and more, I try to stop my swing when my left shoulder hits my chin. If that's plenty of turn for me, then it's plenty for you. If I stop my shoulder turn at 90 degrees while rotating my right hip slightly behind me and keeping my left arm straight, there isn't a par 5 on earth I can't reach.
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I drive it best when I stay "tall" on my downswing. That gives my arms the room they need to come through and fire the club with max speed. I lose the space when I start bending my knees on the way back down. This sets my shoulder line, my belt line and my knee line at too severe of an angle, which makes it impossible to return to my address posture at impact. To keep things properly lined up, I picture a dot on my right hip as I start down. As I rotate, my goal is to keep the dot at the same height and my knees stable. It's worked from Day One.