[Matt]

The left arm is never on plane. The right forearm and clubshaft should ideally both be on-plane at address position. Put another way, the left arm is "steeper" than the clubshaft at address.

When you "pull" on the left arm using PP3, you are stretching the left arm straight. Since the left arm is "steeper" than the clubshaft at address, you are in essence pulling under the plane - "below plane."