Bruce Lee said, "Turn into a doll made of wood: it has no ego, it thinks nothing, it is not grasping or sticky. Let the body and limbs work themselves out in accordance with the discipline they have undergone."
This quote can be applied to piano playing. A doll made of wood can be a pianist that plays without any unnecessary tension. If one would hold out the arm of the upright wooden doll and then let it go, the arm would drop back to the doll's side without resistance. A pianist that imagines being a doll of wood can instantly feel the joints such as the wrists, elbows, and shoulders not having unnecessary tension.
Having no ego means that the pianist is completely free physically, mentally, and emotionally. Ego supresses freedom. One of the ultimate goals of a pianist is to be completely free.
Thinking nothing means having an empty mind. One is focused.
"Not grasping or sticky" can be the physical approach to piano playing, and specifically the attack and release of notes. Having a touch that is like desparately grasping the keys contributes to tension in hand and the body. Unnecessarily holding or clinging onto the keys contributes to a less free sound. For example, a person who rings a bell usually does not continue to hold the mallet to the bell after it has struck. Also, a perussionist usually does not hold the cymbals together after they have been struck. A pianist that does not have a grasping or sticky touch has a natural and efficient way of piano playing.
"Let the body and limbs work themselves out in accordance with the discipline they have undergone" is an acknolwedgement that the pianist has gone through several stages of learning: 1. Learning what notes to play and when to play the notes; 2. How and why to play the notes; 3. Going back to step 1: Learning what notes to play and when to play the notes.
After the first beginning step becomes second nature, the pianist learns techniques, exercises, and various approaches to piano playing. The final step is like what J.S. Bach said, "There's nothing remarkable about it. All one has to do is hit the right keys at the right time and the instrument plays itself.” Step 3 returns to Step 1. The pianist continues following the steps as learning never ends. Like the Bruce Lee Jeet Kune Do symbol, the Yin-Yang symbol continues to rotate.
Also, another meaning is that a pianist who has practiced with much discipline and focus does not have to think about every detail at a performance. The martial artist who has practiced a kick 10,000 times and doesn't need to consciously think about every individual movement. Spontaneity, creativity, and fluidity are very important.
This is one example of how Bruce Lee Jeet Kune Do can be applied to piano playing!