Think of someone that graps your wrist. Now the height at which you present your wrist is a major factor in how good you are able to divert imposed forces through your body (spine) to the ground. Many, many people present their wrist way to low, in which case the entire technique renders invalid (as aite no longer pushes forward, but downward). The next thing to consider is how your body is placed 'behind' your wrist. The contactpoint, your wrist, your center and ground contact are aligned such that you can indeed minimise torque 'spill'.
This off course won't work when you are rigid in either arms, spine, legs. Torque finds your weakest 'link' and overloads it easily, thus breaking your posture.
Hope this makes some sense. Pretty hard to describe properly.
Makes sense. It is good that you are thinking about these issues and presenting them to your students!
Did you learn the "fixation of the upright, rigid spine" as part or element of your aikido?
Then this would be different from what I try to develop.
Sometimes it is explicitly pointed out, "have good posture", or other variations of this phrase. Other times you just learn it by induction watching your teachers, I point out, say Saotome Sensei, shihan of the organization I am a member of:
Youtube link: Saotome Sensei
Now, to a point, if you ignore the orientation of the spine, but pay attention to the shape of it, he is rather stoic most of the time. Occasionally he moves it, but either he is moving it so subtly that it's hard to see, or he's not moving it much on purpose, but either way, me, as a boots on the ground student, sees that, and the message is just "have good posture". And as you move down the hierarchy of teachers, they become progressively more stoic in their movements, not less. Now, maybe if you are a clever monkey, you can reason there are other deeper things going on there that can't be seen by naive eyes, but then maybe we ASU monkeys are not very clever. We are more monkey see, monkey do.