This is a copy of a post I sent to the OP after she contacted me. I thought it might help explain, better, what I was saying above.
Many times we are told to have "quiet hands". We translate it to mean our hands should never move them (relative to ourselves is how we apply it). Actually, we must keep our hands still relative to the horse. When a horse moves, the head and neck are in constant motion and our arms must be as well to absorb this movement.
A good way to get a feel of the arms need for flexibility is to have someone stand next to the horse's head, facing you. Have them hold a rein near to the bit. Then have them pull and release on the rein to simulate the horse's head "bobbing" while it moves. At first the rein will come across your hands resistance to moving at all, causing the reins to snap loose and tight. Then you must soften your whole arm so that it can move with the rein. Remember, you must keep a CONSTANT contact with the rein, not an inconsistent one.
Think of contact as a slight pull back on the rein. How much you need depends on what you need to do to contain the horse's energy. Let's say you need a half pound of "pull" on the rein. As the horse moves you need that same 1/2 pound at all times. If you need to half halt, you briefly add a couple of pounds and then go right back to your 1/2 pound while you register how the horse responded to your extra pounds (half halt).
Your ability to keep that steady, reliable contact through the reins is what it is all about. Try that exercise so that you can see how much your arms must move to keep those quiet hands (relative to the HORSE, now).