What causes the Left shoulder coming up and out ?

[macgolf]

Please help.
What causes the left shoulder to come up and out?. Maybe I should ask how do you keep your left shoulder down through impact?. When studying pro swings I noticed how well the left shoulder stays down and through the shot. Especially the better ball strikers. It "looks like" the left shoulder almost stops moving at impact and the right fires through. Any drills or suggestions on what to practice or where to refer to this in the book is much appreciated.

thanks
Mac.