In the Warring States Period, the State of Qin besieged the capital of the State of Zhao. Duke Pingyuan of Zhao planned to personally seek assistance from the ruler of the State of Chu and wanted to select a capable companion. A man named Mao Sui volunteered.
When negotiations stalled because the ruler of Chu hesitated to send troops, Mao Sui approached him, brandishing a sword. Confronted thus, the ruler of Chu agreed to help Zhao against Qin.
This idiom means to recommend oneself.