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.
Speaking as a professional scientist I'd say that there is too much science here and not enough thought. The use of the word 'tense' in a scientific context is one thing, when using it to distinguish between relaxed and tensed in most (non scientist) peoples everyday lives it seems reasonable to work with language more easily understood by most people. If you want me to I can technobabble this stuff for you and relate it to points of anatomy, physiology etc etc. However this will likely not help you or anyone else improve your performance of this exercise.
Remember, I doubt anyone would claim that these are supernatural powers and if they do they're probably a bit strange in my book. Arguing over point so scientific technical language isn't particularly helpful either.
Speaking of science. I have to go and do some or the boss will wonder what I'm being paid for. Back in a few weeks.