Empress Kira wanted answers which this guy had but he had refused to give them to her. She was not going to beg the answers from him and that is why she took the step to punish him and make sure that the pain made him talk. She cruelly forced the guy to talk using her foot domination. He did and regretted why he had refused to give her the info.
Princess Kira and mistress Sofi did not like how their slave behaved. They wanted him to change so that he became what they wanted him to become and to behave how they expected him to behave. So they used foot domination to show him that if he did not change, he would have it rough and he would have to endure a lot of pain as well as humiliation. He chose to behave like it was expected of him.