Our physical motion is controlled by two systems, voluntary pyramidal tract system and involuntary extrapyramidal tract system. We can't control nor train the latter easily. In keying, both sysytems are involved. We could hardly control it sometimes. All we instantly could do is the voluntary movement and the basic muscle tension control. The muscle tension is controlled voluntarily as well as involuntarily. The voluntary portion is important because it is not often being conscious but is controllable.
In keying motion, the arm and hand should be placed in mildly flexed position. It is the most spontaneous and restful position and it means the muscle tension could be least with that posture. We make errors in keying mostly when transiting between dash and dot, that is, changing to and from between dash and dot in code of each character. Our fist should obey the order from the CNS immediately and correctly in this motion. When the fist is not in resting position, we should put extra tension on appropriate muscles to move the paddle or key,. It could cause erroneous movements of fist. As told before, it is not the only factor causing errors. But it is important to be conscious of that because we could correct it easily.
As I extended the right arm and made more mistakes in sending, I realized it. I could advise to relax the arm and hand in restful position of mild flexion to anyone who is feeling troubled with sending errors. Yes, as if softly hugging someone important to you.