I am a non-musician. But I pay attention. What he is trying to do in the beginning, and I didn't understand it, was listen to music that he was imagining him playing the drums along with. Which he then turned into a masterful accompaniment of the initial music. I had to go back after and listen to some of it and watch again because at that point I understood what he was doing. Just initially I see a guy listen to music and sounding like he was memorizing the music to maybe replay the music. That was quite interesting as well but I was wrong. He was actively creating in his mind while he was listening to the music.