i would look at the 48th demo of Endo sensei, at the beginning where he dealt with ikkyo then iriminage.
When you watch this video of a seminar in Washington
from the time where I zoomed in to the end of the clip, you may get an glimpse, of the use of the body or espacially the spine in this way of aikido.
If you don't want to watch the whole scene, please at least look at 2:05.
Is this more interesting?
And as far as I understand and experience it, this flexibility is always there. Even if you don't see it.
I think, an upright spine is ok. But it should be soft and flexible and should be able to "answer", should be "responsive". The whole body should work this way. At least this is, what I try(!) to achieve.
Aahhh very very interesting! Thank you all for your statements. I have to think and that's enriching!
Believe me, I'm really looking at these videos and trying to see it, but I'm just not seeing it. At 2:05, I see the hips (some lumbar) flopping around, I see the skull (cervical spine) flopping around, which is indicative of a pliability in the area, but the thoracic looks fused, and when he's actually moving, it all seems stiff, fused. There is pliability, and then there is intentional mobility. It's one thing to allow outside forces to drive you into a position by not resisting it, but another to actually drive yourself into it. They don't necessarily cross over in terms of learning.