[Matt]

We're talking about stretching below the clubshaft/right forearm plane (which should be a single plane). We're not stretching below the plane of the left arm - we're stretching the left arm which results in a stretch below the clubshaft/right forearm plane.

The naturally "steeper" angle of the left arm (in relation to the clubshaft/right forearm) results in a "below the clubshaft plane" stretch when you stretch it.

I know that's a wordy answer...so let me know if you still have questions.