In the Warring States Period， the State of Qin besieged the capital of the State of Zhao.
Duke Pingyuan of Zhao planned to ask the ruler of the State of Chu personally for assistance.He wanted to select a capable man to go with him.
A man called Mao Sui volunteered.
When the negotiactions between the two states were stalled because the ruler of Chu hesitated to send troops， Mao Sui approached him， brandishing a sword. At that， the ruler of Chu agreed to help Zhao， against Qin.
This idiom means to recommend oneself.