Tap your head with your left hand and rub your stomach with the right one, and you’ll sense the mutual influence between the right and left arms. This we call bilateral transfer—a dialogue between the two sides of the body on matters of position, movement, tension, relaxation, and balance, all of which affect the body’s overall coordination.

The legs also affect the arms, and vice-versa. Play a fast, loud passage at the piano while holding your feet off the floor. If the active support of the feet and legs is missing, the arms must work much harder. This is quadrilateral transfer—the interplay of energies between all limbs.

The dialogue between the left and right sides of the body, and between the upper and lower limbs, never stops. Like all dialogue, it can be a collaboration or a fight.

To get a fight going, hold a heavy paperback in one hand and a light bulb in the other. Each hand has a specific job to do, but each hand confuses the other and is confused by it. One hand “wants” to relax, the other “wants” to firm up. Their opposing intentions get crossed, and the body and brain go haywire.

Try another experiment. Write a short sentence by hand on a piece of paper. Now write it again, and while writing tug at your hair with the free hand. Make the tug be strong and rhythmic. You may be surprised at what happens to your handwriting. (I did this experiment with my wife, and her handwriting actually improved, becoming bolder and more legible. She did mis-spell a word or three, though.)

Because of bilateral transfer, musicians sometimes misdiagnose their technical problems, becoming convinced that the left hand, say, is to be blamed for some technical accident when in reality it’s the misuse of the right hand that causes the left hand to go awry.

Suppose a cellist is struggling with a tricky passage that challenges her left hand: a trill followed by a large shift along the fingerboard. The left hand is fast and busy, doing different things in quick succession. Meanwhile the bowing arm does something simple and steady. The average cellist focuses on the busy left hand, giving it thought and care. Naturally, her thoughts are coated in emotion: eagerness, worry, impatience, anger. At the same time, the cellist takes the right hand for granted, assuming its role is minor. The passage remains frustratingly difficult, and the cellist puts ever more energy into the left hand and involves her neck and shoulders in the effort.

But if the cellist changes her focus from her left hand to her bowing arm, bilateral transfer comes to her rescue. The right arm proclaims, “My gestures are easy, firm, intelligent; my contact with the string is stable; I have a lovely connection to the back, the pelvis, the legs, the feet, the floor.” It’s a message of intelligence and comfort, with a positive emotional charge. The left hand receives the message, absorbs it, lets itself be influenced by it—and acquires some of those universal qualities (strength, contact, connection, comfort) even though its specific tasks are different from the bowing arm’s simple gestures. As if by miracle, the passage suddenly becomes much easier to master.

In sum, bilateral and quadrilateral transfer are both potentially harmful or constructive, depending on how you go about it.