In shadowboxing you practice self-expression with no inhibiting factors present. You learn the correct moves. You form a “mental map” that is retained in memory. You create a broad, general, flexible map. Then, when you face a crisis where an actual menace or inhibiting factor is present, you have learned to act calmly and correctly. There is a “carry-over” in your muscles, nerves, and brain from practice to the actual situation.
