[Matt]

I think (and i'm no expert) that the left arm is off-plane at address not because of extensor action, but rather because of the nature of the "left arm flying wedge". Which is properly created when the club is placed in the heel of the left hand which creates an angle between the clubshaft and the left arm.

I don't wander into the "advanced thread" very much, so be gentle on me!

You're right, Trigolt. Extensor Action doesn't create the off-plane left arm; the left arm being off-plane is just a consequence of setting up correctly!