A while back, a kind professional pianist told me how slowly she had to practise difficult material - so slowly, that she would let her hands come to rest so she could fully grasp the correct position for them. I've been practising Mozart's variations on Ah, vous dirai-je, Maman? and wondering why some parts seemed impossible, and others jerky. Her advice keeps returning. Slow really is, at times, dead slow, so that you can see exactly where to hold and where not to, and get the hang of fingering that goes against your instincts, but is necessary for a note a little further on.
Why are these things not explained clearly enough for people to understand them? Similarly, Fanny Waterman, while telling her students on youtube that she is "not the boss" insists on certain fingerings in her arrangements - say, of Christmas Carols - but does not explain why. If that doesn't make her the boss, I don't know what does. Some fingerings are unusual - eg switches between thumb and first finger when others are available - and it's not obvious why they are used. As a teacher once put it to me, you have to work it out for yourself. Time-consuming, when a few introductory words before the piece would suffice to explain the issue.