When I first started Aikido it was doing Ki Society aikido and lots of attention was given to the Ki Development exercises. At that time I set aside my training in physics and just learnt the methods that produced the results. later on i integrated my physics understanding to explain this mysterious Ki stuff.
Yet still the reason behind the Ki testing alluded me, today though through the prisms of what people are trying to do to understand internal strength and my understandings from biomechanics I begin to see what Tohei was trying to do and its time to revisit the Ki Test again I think.
A misconception might that its a test and therefore it is about passing a test. Instead rather its a tool for helping each other to learn something. The great failing that I saw of the method was that it became an egoic activity to assert dominance, as there are planty of ways to 'game' the result if you are testing...or so i thought.
Armed with the results of 'toppling' i see now that Ki testing is more about the relationship between the person trying to pass the test and the person doing the testing..e.g. in unbendable arm initially its about trying to align the posture of yourself, relax the bicep, not react to the tester to pass progressive levels. But now to pass at a higher level setup the topple in the testor so their power never even enters your arm and you have something pretty powerfull
To the right is the Ki Testing component of my Ki Society Shodan test from the 90's looking back at it I can see it a fresh and its purpose. Here is more on Ki testing