A couple of points:
(1) Indeed, many times during the instruction process I found myself thinking “I wish I could just experiment!”.
(2) However, there were at least two specific pieces of verbally communicable non-obvious information that proved to be crucial: (a) the idea of putting one’s finger on the side of the book near the spine to detect the pressure of the blade and thus determine that the book is in place; and (b) the idea of preventing dislocation of the book simply by cutting sufficiently quickly. I’m not sure I would have discovered these things myself very quickly at all.
Even more than that: it requires the ability to communicate that conscious knowledge to the other person (thus, a two-place function). Each of us has our own internal “programming language” that determines which words or thoughts correspond to (e.g.) which bodily movements, and furthermore we tend to have our own specific repertory of bodily movements that we’re used to making without thinking, which may not be the same as another person’s. A teacher has to be able to bridge this gap—which, in particular, requires awareness of its existence in the first place.
(Analogues of this hold for less physical tasks, e.g. doing calculus problems.)