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!