Wu Wei, Chinese for "not doing," sometimes is mistaken for passivity. In fact, it refers to spontaneous and effortless action. For example, an athlete whose body flows through his surroundings without calculation or hesitation is demonstrating Wu Wei.
Wu Wei is an important principle in the martial arts. A master's actions flow spontaneously in response to his circumstances, with just enough force -- not too much, not too little. Wu Wei is action without desire, motivation or intention.
See also "Wei Wei: The Action of Non-Action" by Elizabeth Reninger, the About.com Guide to Taoism.