I watched his bowing arm as it moved across the strings. Something happened. I couldn't quite catch it, but it seemed out of place. So I asked him:
"Have you got some idea of transitioning your arm as you stroke down?"
"Yes! Oh yes, yes, yes. How do you know that?"
I saw it. And I guessed that he was thinking something, overriding natural movement with a pre-conceived notion - I must control this moment, I must do something. I was teaching a group class in BodyChance.