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.