the term 'unbendable' arm I have always thought is a misnaming of the mind body state that we look to achieve.
I know my unbendable arm is only unbendable to the person who is trying to bend it. It is totally bendable by me, even when they are doing all their huffing and puffing trying to bend it. It is not locked or tense, it might feel like it to them, but not to me. It goes where I choose it to go, not where they are trying to put it. If I was locked and tense, I would not be free to move.
All of these states are easy to demonstrate, as with Ron's ki exercise above. Never so easy to write about.
I agree. My point was that you can lock the joints and still it will work also. That's what Ron was practising, locking joints and tensing yet still having it work.
Of course it will impede your freedom to move done this way but he is saying he had never done it this way before and was surprised it worked. I would say it's less efficient when movement is concerned and also not as fluid. As I said, I used to call it straight Ki and as such found uses for it, especially when using hand as a spear for example or even a finger come to that.