Playing shakuhachi requires a balance of concentration and mindfulness.
By concentration I mean attention to the sound you're playing, noticing every nuance of its pitch, tone color, dynamics, etc. It is especially important to notice how the sound changes over time, how the very tiny differences from one moment to the next are the essense of the music.
By mindfulness I mean openness to whatever else is going on: to the sounds of traffic out the window, to the sensations in your body, to the attitude of listeners. All of these, too, are part of the music. Shakuhachi music should have a very open weave, full of ma, never suffocating the world around you. In playing shakuhachi, you are really joining an ongoing symphony.
I believe that skillfully playing with the balance between concentration and mindfulness is the essence of musicianship. This is something you can work on every single moment you practice shakuhachi.