I said I would talk about using your arms, but I’ll start with the hands first.
You see people doing different things with their hands- maybe clenching them in a fist or deliberately holding them open so their fingers stick out. I’ve heard a piece of advice several times about the hands that I agree with.
I think it works best to imagine that you are holding a large potato chip in both hands. You wouldn’t want to crush the chip, so naturally you won’t clench your fists tightly, which might cause unnecessary tension. Also, this will keep you from opening up the hands the way many runners do. The open hand position can also lead to tension, because one has to flex the muscles of the hand to open it. The potato chip hand position ends up being the most natural, least forced option we have.