The author divides your self in two. A self that reasons and (over)thinks. And a self that just does. The inner game is to let that second self do its thing, and letting the first self get out of the way. Useful in tennis and likely in other physical activities (like making music).

It made me think of Yoda (do, or do not - there is no try).

It's a small book and I think I'll need to read it again.