I don't know... Maybe try to lure the dog backwards until they fall and mark it? If you kept the treat closer to their skull and lured backward, the dog might follow it with their nose instead of their whole body. Hmm...
Well, it was very cute. I doubt that's one I will ever conquer with Kaki.
I was thinking the same thing. Perhaps the dog tumbled that way 'naturally', and it was simply captured.
If not, ... say the dog went down in a typical sideways heap to begin with, and the trainer wanted to make the action less abrupt and more linear ... maybe she started the dog in a corner of the couch etc where the dog could only fall straight back a limited amount, and she could refine it that way ? I honestly don't know, just guessing on that of course.
It would be interesting to see and/or hear the cues she gives to the dog. That might provide some minor insight as to how she went about it.
BTW I do agree completely with the concern for the dog's back, first and foremost. Not something I would advocate for the masses, really.