Isn’t this just running the manoeuvre through your head?

That is one part of it, the best known and the smallest. Going through movement sequences in your head — mental technique training — helps with a tack or hoisting a spinnaker. It does not help when you are shaking at the start, when you check out inside after a premature start, or when after eight hours of rain you no longer want to make a decision. That needs methods that work on tension, attention and inner dialogue. Running it through is one tool in the box, not the box.