George S. Ledyard
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.
It is flatly impossible to straighten or bend the arm "without tensing the arm muscles at all". Tensing the arm muscles is what makes the arm bend or straighten. This is not only true with someone providing resistance but also when one is floating freely in a non-gravity environment. The arm does not voluntarily bend without the application of muscular tension on one side of the joint sufficient to overcome the resistance to bending, no matter how small that resistance is.