To me the paint in the Instagram link is just going around on the forehand, like a lower-level HUS stock horse. I don’t see the energy cycling through.
I’m surprised Manolo Mendez hasn’t come up in this conversation, because he is an advocate of working the horse in a FDO position for a significant amount of time (he says “short the body, long the neck”). However, he doesn’t take them so low for any significant amount of time, and instead aims for the eye being at or a little below the level of the point of hip. The horses develop a beautiful bascule. If you do a Google image search, you’ll also see him working horses in a more advanced frame, and using piaffe in hand to develop strength.
http://classicalway.pl/media-spolecznosciowe-co-linia-grzbietu-mowi-o-koniu-i-jego-jezdzcu/
Now doing a Google image search for A2R, in comparison, with the horses much lower in front, the “V” formed by the front and back legs is farther back under the horse, so they’re not supporting and building with the hind end as much.
It shows in the video still of his horse Bailador, and it shows on the paint horse in this link (at an A2R) clinic:
http://www.classicalfoundationhorsemanship.com/correct-stretching.html
I have a mare that is a little downhill, and has western pleasure breeding on her sire’s side. She would gladly go around in a low frame all day and never get “through”. It took a lot of diligent work building her strength before she was strong enough to stretch properly and continue to step through with the hind legs. I would not accept a horse just burying its head as being a “good stretch” IMO the ability to stretch is built out of strength and elasticity, and the right to do a deeper stretch has to be earned. Also, it should only be done for a few strides at a time to start, and then built up from there, otherwise the quality will fail.