This is such important advice, and so often exactly the endless loop of discussion I have with clients. They will tell me that another trainer said they were sure horse XYZ could surely already do ZYX, and I why I am not giving it a go. So we, the people teaching horses, should ALL scream that off the roofs of wherever we happen to be!;)
Very interesting. It reminded me of a horse I had when I was not very knowledgeable yet. My horse some how got the message of what I wanted but I didn't even know myself exactly what the aides should be. So everything was fine until it wasn't. I wish I had at that time someone as experienced as you to explain what happens psychologically in the horses mind and body.