Audiation
The ability to intentionally imagine pitches
Audiation is the skill of intentionally imagining sounds. For example, it could be imagining what a melody consisting of a certain sequence of degrees sounds like, or singing it, purely based on your knowledge of the degrees and the starting point. It is a form of relative pitch and a pillar of ear training.
This ability complements the primary ear training method used in NoSheet, in which some musical thing is heard and you must find it on your instrument — instead, it shows you how to find it, but not how it sounds, and you must figure out how it sounds.
If both abilities are strong, you effectively have a version of your instrument entirely in your mind, that you can play at will and that is connected to the physical instrument. This is how you become one with your instrument.