Thanks for putting up that video, it is a very good one for beginning fly casters. Mel Krieger's videos are also very good.
With all respect, because I know you are a good caster, I think this advice is the worst you could possibly give to someone new to casting:
It's all in the wrist, not the shoulder
As the fellow in the video above said,
It's all in the forearm, not the wrist.
By that, he is talking about the arm movement only, not the rest of the principles involving the cast. But the basic forearm movement and stop are the principle building blocks for the cast.
Wrist snap will come into the picture later - after the caster has learned the fundamentals and has started to develop his own style. Almost any wrist movement whatsoever in new casters - especially if using the thumb on top grip - will result in collapsed backcasts or no loop on the backcast at all.
One thing I would suggest to you Jwitt, is to strap the rod butt to your forearm with a rag or velcro or something and to use a strok
e like you are trying to flick paint off a paint brush or to throw water out of a cup behind you on an upward trajectory.