Concubines as in the emperor’s court mistresses.
Here’s the Sun Tsu story:
Sun Tsu goes to a royal to get a job teaching his military force. The emperor asks if he can train anyone in military matters, when Sun says yes then the ruler says fine show me using my concubines.
So Sun Tsu forms up 2 companies of concubines and asks the ruler who the most senior are and if he has total authority to discipline when they fail to follow orders. The ruler ids his 2 favorites and says yes he can discipline.
Sun sets the 2 favorites as company commanders and then goes through training both companies on the basics of close order drill. He instructs the 2 “officers” on their duties.
Then once he’s convinced everyone has been trained he tells the 2 “officers” to drill in front of the ruler. The girls break down giggling. Sun stops the drill and tells the officers that they are required to discipline the girls when they fail to follow the commands properly. Then starts them drilling again. Again the girls break down giggling.
Then Sun orders the 2 ‘officers’ who failed in their duties beheaded. The ruler is upset, but Sun reminds him that was given authority to discipline in order to show he could train them.
The executions proceed and the next 2 senior girls are promoted to the company commander position. They are ordered to drill again and both companies perform flawlessly.
The emperor is still upset about his loss, but so impressed that Sun can get a bunch of court girls to drill so well that he gives him a job as an advisor.
It’s the one story I remember the most from a semi-history/semi-biography I read about 35 years ago.