What helps me is to think that I am holding a glass of water. I know you usually only hear this for keeping your hands up and straight but I use it to keep my hands Still too.
So, Imagine you are holding a glass of water. If you hold the glass to loosely the water will spill and your hands will go all over the place. If you hold it to tightly (being stiff in your hands and arms) you will equally spill the water. So you need to be in between where you’re steady but not stiff.
I don’t know if that made any sense. It took me a while to get that in my head because I used to be too stiff.
I think you shouldn't bother too much on keeping your hands completely still. If you focus too much on not moving your hands at all you might be bit stiff which isn't what we are looking for.
Instead focus on having supple arms and be able to follow the movements of your horse.
The most important thing is the connection you have between your horse's mouth and your hands.
You should be able to have either a hand of action or a hand of orientation. The hand of action would be correcting et rectifying a movement and the hand of orientation would be a hand of support, communication which keeps the contact with the mouth.
I hope my answer helped! :)
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