[tongzilla]

Again, considering the starting point is always the Turned Shoulder Plane, how can the rear shoulder be moving on a TGM defined plane at startdown unless it is the Turned Shoulder Plane?

Like you said it starts at the TSP. There are many plane angles that can go through the same point. Some players move their right shoulder to a steeper plane immediately at startdown -- i.e. above the TSP.