I guess you haven't been around long enough to follow his content.
It's not just "the double move", it's his concept of being a slick car salesman, that (mostly) gives Ill advices and have next to no clue about biomechanics (I'm biased here.. can't stand the dude)
He's been putting emphasis on that it's the off arm (and back leg) that drives the swing and not a solid brace. That you need to ACTIVELY fire the off arm and turn the rear leg in to get a rotational throw, when in fact, the two things are biproducts of a great brace AND the right posture, timing and sequence etc.. oh and a lateral shift.
With mcbeth in the slowmo, you can see that he shifts from behind and do not fire the off arm to start the throw.
I might be wrong and eat my words, bit whilst the offhand definitely helps to accelerate the throw in some manner, I am a firm believer you and many others thinks of it in a wrong way with the "stretch reflex" in the upper back and how it's a BIG boost to distance..
In my opinion it isn't and it's more a byproduct of correct sequencing, timing, posture along with the right swing thought.