The term 'unbendable' itself does not correctly describe the end state that is being trained for. It is true that if someone is testing my arm using strength, then what 'they' feel is that my arm is un- bendable, but I can choose to bend it as I wish, therefore it is only 'unbendable' to them.
One should not only be able to keep it unbendable under pressure from a partner but bend it and straighten it without them being able to stop you. This should be done without tensing the arm muscles at all. If one has this type of energized flexibility, one has what is needed to execute technique without either clashing or collapsing which is what many folks do.