During the Spring and Autumn Period, the armies of Qi and Lu faced each other. When Qi beat the war drums for the first time to signal battle, the ruler of Lu wished to attack immediately. His advisor, Cao Gui, advised, "We should wait until the third drum roll, sire."
After Qi had beaten the drums three times, the Lu army attacked and achieved victory. After the battle, the king asked Cao Gui for the reasoning behind his unusual advice.
Cao Gui explained, "Fighting requires spirit. The enemy's spirit was at its peak with the first drum roll, diminished with the second, and completely exhausted by the third. We attacked when their spirit was spent. That is why we won."
This idiom has come to mean accomplishing a task in one sustained, vigorous effort.