Consider the empo you use to get a new piece up on its feet. Many people get tempted to play up tempo sooner than the fingers are able to.
But you have to make the fingers go as slow as the brain needs to go through all of the thought process. Only then can the fingers play accurately.
When you play faster than the brain can comprehend, the result can only be messy. Repetition at the same tempo won't make it better, but worst, because of the muscle memory of the repeated mistakes.
When a piece is played as slow as needed for the brain to think, the result will be consistency because of the team work between the brain and the fingers.
Next time you hesitate to practice a piece twice slower, consider the actual speed your brain has to think at at that tempo, and you'll realize that it's not nearly as slow as you think.
Picture from http://www.learn2rap.com/
Posted by Geraldine at 10:36 PM