What is the most important thing in connection with the training of the horse?
It is to teach the horse to work properly over its back. This means that the horse must learn to work with its back up/lifted.
The strange thing is to search the web, so there is not really much to find about the subject. And often when you finally find a good text about it, the pictures do not fit. At least not to the idea I have about how it should look.