substitution technique
A common technique used by ninja to get out of trouble. Just before being hit by an attack, the ninja disappears in a cloud of smoke, leaving behind an inanimate object in his place, most often a log of wood.
Sometimes the log will be left wearing the ninja's clothing, while the ninja is left nude or nearly so.
While popularized by Naruto, this trope is not specific to that series, nor did it originate there.