OK, so the "ki rank" corresponds to ki development, but what does that mean? How do they test ki? Does it have anything to do with things like the unbendable arm?
In my less than three years experience in the Ki Society, here's what I can say: Ki testing and advancing in ki level is a requirement as you reach certain rank. As you advance in your grade in aikido, you are required to attain a certain level of mastery in extending ki and not cutting it. By this , I mean being able to maintain coordination of mind and body during the test (hopefully you apply this in your daily life which is the true benefit).
Basically, the test involves distracting you at what ever you are required to do during testing, unbendable are being one of them (there's a lot more).
First level of testing test for extension of ki without hesitation right after a waza. If you are extending ki throughout the waza, you should be immovable when they do the test right after the waza.
The second level of testing involves slight hesitation. This requires continuous extension of ki even after the waza. This is a harder test because this goes beyond a physical test, it a test to see if your mind can be disturbed by the slight hesitation. Normally if you see a hand coming in to push you, the tendency is to prepare for the push so you lean slightly forward right? Well, this is mind being disturbed, ki extension being cut. The result is an unstable posture- easy to push. In this test , if your mind is not disturbed by the test and continue to extending ki, you will remain stable and immovable.
The third level is an even harder test in that the tester him/her self will extend ki through you to move you. Like the second level test, you have to extend ki constantly. This time, you are also required to not receive the ki of the tester. This is a total mind job! Normally the tester already know if you are extending ki during his approach. If you cut ki, the test is over even before the tester touches you!
The key thing is (pardon the pun) for you to exted ki all the time. To be able to face any problem: school, work, home, while driving, while being attacked, so you can deal with them the proper way without getting upset.
This is what I know from my limited experience. Corrections are welcome.
My ki rank is at the first level and I'm preparing for the second level on my 3rd kyu aikido testing.