Paul,
I think it is more important that the arm is relaxed than forced into an unnatural position. I have studied many theories on this subject and am of the opinion that a relaxed arm even with a slight bend is desirable (Not a collapsed arm though).
Here is an article from the Bio-mechanics site:
"The left arm forms the radius of your swing arc. If the left arm bends during the swing it will force the clubhead to move out of position forcing you to make adjustments during the swing to bring the club back to square at impact. The goal is to have the the left arm in the same position at impact as it was when you addressed the ball. To make sure this happens start with the left arm straight at address and concentrate on getting it square again at impact. It's almost impossible to not bend the left arm at all during the swing. Some bending will occur naturally. This natural bend of the arm will correct itself on the downswing due to the centrifugal force created during the swing. The key is to not let the arm bend any more than it naturally would during the swing."

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Best Regards
Brian

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Funny o'l game!