What does “singing inside of the head” mean?

The most important thing at a recital is the music. It is not about how well you play or if you can play without an error. What’s important is fully expressing the music you practice daily and think about to the audience. In order to do this, you need to start singing the music in your head without stopping from the time of full rehearsals; even on your bad days. If the music in your head doesn’t stop, the music you play will keep flowing even if your hands accidentally stopped for some reason. In contrast, if your hands kept moving but the music in your head stopped, music won’t flow. Moving your finger by itself is nothing more than physical exercise. Just like astronauts go through a lot of simulation trainings, sing the music in your head the way you want to express it when you practice. Singing in your head takes a lot of training and it’s not something you can do suddenly at the recital.