Unbendable arm done from such also. Putting one arm out and having one person push your hand trying to push you backwards with all their might is another example where arm is locked and tense.
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.