Hmm, as I said before, it's a little hard to explain.
Have you tried unbendable arm in the way Dallas described? Once you've tried that, it becomes very clear on a sensory dimension - kind of thinking of your arm as an unjointed, single unit, like a firehose spewing water at high velocity. I think your dojo mates most probably know this exercise already. Get some of them to test you and you'll find that the more completely you ignore their efforts to bend your arm the more difficult it will be for them to do it. Your arm mustn't be a limp noodle, but if you concentrate on the person trying to bend your arm, you'll unconsciously use your muscles to resist, thereby defeating yourself.
Once you get used to trying this 'trick' it becomes much more natural to apply unbendable arm to your normal technique.