JW

I have some thoughts on this matter though they may not be in the original spirit of the thread. Mike, I know you saw Sunadomari's AJ Friendship Seminar demo/lecture. This was a BIG topic for him, he actually spend a lot of time discussing it.
I took away this as a basic understanding:

From the point of view of outward force from you, the finger extension is involved in the mechanism of your generation of strength. There are 2 contexts in that demo that you will see Sunadomari's hand extended: when he shows you what is "wrong," and when he directs ukes around him, by using some outward force. So why is one wrong and one is actually what he does in application?

When you want to put out outward force (obviously it will be into a direction where there is no ability for uke to resist) then this kind of expansive mechanism is used. But then there is the receiving of incoming force, which is a different matter. This is where Sunadomari's harping on what is not right comes in. If uke is coming in with strong force, then receiving the grab correctly involves a mechanism marked by hand closure, rather than hand opening. He shows this many times. It seems to be the case that by fully and actively receiving the incoming force, his influence can 'enter' into the attack. He shows how using strength (with the hallmark extended fingers) in this case is exactly the wrong thing to do, and you just get pushed around.

I took it like, if someone gives you the 'yang' tomoe from the diagram, you complete it by mating it to the 'yin' tomoe. But vice versa when they are weak-- in that case they present only the 'yin' tomoe so you fit that puzzle piece with the correctly matching 'yang' tomoe.