As far as unbendable arm, you simply "extend" the arm rather than "resisting" the bend. People apparently find it easier to "extend" the arm if they visualize ki flowing "forward" out their arm. At least Tohei felt that it was good to describe it that way. But the truth is, you just extend the arm.
Now you can get into all kinds of things about how the body maintains its balance while you do that, but we've seen an example of where someone got Tohei off balance in that very thing so I don't see a need to go there. As far as the "unbendable arm," it's just extension.
OK, I have a friend visiting at the moment and I just walked out in the living room and asked him to just extend his arm. Then I pushed down on it in the approved manner. His arm bent. I followed your directions, so I don't know what went wrong. You got any ideas why that description didn't work?