Re: opening the joints
It's not "expanding" the joints that he shows, it's the manner in which the joints are opened and the palpable change in soft tissues around the joints. It's part of what he works to teach as "good form" in aikido and in his bagua training. In the context of aikido he sees it as subsumed within "extension of ki." In physical manifestation it's similar to the zhan zhuang demonstration of a Canadian yiquan teacher when he's working on six-directional forces. Both also talk of opening of the joints in the context of the Chinese term "song," looseness (not wet-noodle relaxation), and how the "openness" of a joint is affected by tension on either side and elsewhere in the body. A lot of their hands-on correction and teaching could be called "tension management."
It may well not be what you're referring to as "extension of ki." "Ki" is a big topic, and I'm not prepared to delimit precisely what Tohei or Ueshiba had in mind when they used the term. I just got done with Kenji Tokitsu's treatment ("Ki and the Way of the Martial Arts") of the set of ideas with the referent "ki" . . . and I'm going to have to go back through it again in light of some of the discussions here. The set of meanings embraced by the Chinese "qi" and the Japanese "ki" overlap, but the terms are not completely synonymous.
But I don't want to divert from Gernot's topic . . .
"Opening the joints is extension - in all directions. Opening all the correct joints (I'm not sure if there are some that remain closed) creates one point and weight underside. [snip]
Think of extension of limbs and fingers as the way to open the joints rather than making a shape which needs to be preserved with hardness.
The same applies to the neck and back, hip, knee, ankle and toe joints, without whose action it will be impossible to really "sink" underneath the partner without moving at the point of contact and "lift" (or in some cases "sink/collapse") uke before the visible movements begin.
The trick is in keeping the joints open while moving the limbs in gross external motions (vertical changes via knees, rotation and lifting/sinking of the arms). For such, the limbs must be connected and powered by the center. Without the open joints, I think the power of the center cannot reach the extremities and control their motion precisely."
I think that the aikido teacher and the yiquan teacher I mentioned above might agree with your description, Gernot. I'll be seeing them again in January and February when I will have some free time for travel. I'll do my best (which ain't much) to physically understand what you describe and run it by them. They have real skill, so it might be an interesting comparison. But Mike's right, it's important to be clear and consistent with the terms we use to write about these things. I think I understand what you mean by "opening of the joints." I'll leave "extension of ki" aside for another thread.