This is one of the most elegant and cogent phrases I have ever known. It explains a huge overreaching concept in two words. Mr. Werner and I are roughly of an age, and he has justifiably received more recognition than I. We do share the perspective that comes with age, and part of that perspective is that we begin to know what we don't know - and we begin to know what is, and what is not, under our control. The title of the book describes a state that we can never truly and completely attain, but should be a lifelong goal - and which be can be realized in incremental steps.
Effortless mastery is, in my opinion, a simultaneously-occuring internal and external state. As in most things in life, the outwardly-appearing state is much easier to achieve than the internal state. That doesn't make it casual - music is an act of communication, and what we put in the air determines what we share.
In one sense, the simplest definition is making sure that everything you present is under control. That generally means one of two things - either it's an actual sequence of notes (a "lick", if you will) that you have gotten under control, or a musical concept (a melodic or rhythmic motif, to name but two) that you are able to fully auralize in the moment. (My spellcheck threw up this metaphorical arms at the word "auralize". It's the sonic equivalent of "visualize", and it's hearing things in your head. You should already be doing it all the time.)
Full disclosure - I don't like licks. They're lazy. You get to a spot in piece of music, and you make a conscious decision (at least, I hope you do) that "I'm going to play these notes for the X number of beats - they should fit". See, it's that "should" that bothers me. If you can't auralize the idea start-to-finish in the spot where you want to use it, is it going to be "mastery"? I think not.
So, where are we? Let's face it - we're in a place where, quite often, what you play may not be:
• the hippest thing ever heard
• the line that'll make the posers snap their fingers
• New Frontiers In Jazz
...but it'll be something that is totally under control, and will sound effortless. Believe me when I tell you - this is what you want. If you try for the uberhip line, and it goes south, the listeners won't applaud the effort - they'll remember the fail. It's just human nature; it's how the human animal works.
There's another wonderful phrase that come into play here: "Don't let your reach exceed your grasp". DON'T - I'm shouting here - DON'T play something that's not under control. Play who you are now. You'll be that player soon enough. And what will you be if you do this? You'll be the cat that never makes a mistake. And that's who they'll want to hear.
Some of you are saying, "We only learn from our mistakes. We'll never get better if we don't take chances". And I don't disagree. But I would suggest that you make those mistakes and take those chances when you're practicing. Yes, the situations are different, but, as I said, music is an act of communication. Focus on the fact that it's an ACT. When you practice, ACT like it's a performance - and note where the mistakes happen, and correct them.
"Effortless mastery" is a state of mind - a state of mind that only can be reached after a fair amount of practice, a fair amount of performing and (most importantly, I think) a fair amount of self-examination. Know what you can and can't do, and think about what you can and can't present. Think about communicating effectively, and make your choices - and never forget that, with each day, your cadre of presentable things grows - making you more of an effortless master.