Rob and Tom both talk about the importance of the trail arm at setup and during the swing. Check out this video as it supports what Tom teaches, just in a slightly different approach. The "losing the arm wrestling match" to start the downswing is very easy to both feel and remember. Most of us are too steep, or we're "winning" an arm wrestling match to start the downswing.
https://www.youtube.com/watch?v=8yOZ1iMEFKo&t=615s
Yes - that's a good lesson on "external rotation" of the trail arm and shoulder, and when done correctly it puts the Hogan "Magic Elbow" on autopilot.