This mistress wanted to make sure that she was dealing with a slave who was worth it. The mistress felt that the best way to test him was to ball bust him. That is why she went out of her way to trample him in the nuts and kick him there too to make sure he felt pain. He was forbidden from crying and the mistress made sure he endured the pain without making a sound.
This mistress had a policy that she would not sleep with married men and so she was pissed when this married man insisted that he was the best thing to happen to her. She was pissed at him and she had to dominate him and teach him not to cheat on his wife. So she kicked him in the nuts to teach him a lesson he had never been taught before and he never bothered her again.