Deep practice is built on a paradox: struggling in certain targeted ways—operating at the edges of your ability, where you make mistakes—makes you smarter. Or to put it a slightly different way, experiences where you're forced to slow down, make errors, and correct them—as you would if you were walking up an ice-covered hill, slipping and stumbling as you go—end up making you swift and graceful without your realizing it.
The reason, Bjork explained, resides in the way our brains are built. “We tend to think of our memory as a tape recorder, but that's wrong,” he said. “It's a living structure, a scaffold of nearly infinite size. The more we generate impulses, encountering and overcoming difficulties, the more scaffolding we build. The more scaffolding we build, the faster we learn.”
The trick is to choose a goal just beyond your present abilities; to target the struggle. Thrashing blindly doesn't help. Reaching does.
When you hit that backhand (or play an A-minor chord, or make a chess move), an impulse travels down those fibers, like voltage through a cord, triggering the other fibers to fire. The point is that these circuits, not our obedient, mindless muscles, are the true control center of every human movement, thought, and skill.