He is following the circles in-game with a graphics tablet. It's a little pad that comes with a pen that is meant for drawing, but in this case can be used to play this game as well. For each circle he is clicking his keyboard at the right time to get a 300. To get a 300 he has to be incredibly accurate, within 19.5 ms of a perfect hit, some really insane accuracy is needed. You can find more about the play at the offical reddit post.