Lady Kimah wanted to dominate this guy and she did so using her dirty feet. She wanted to do so because the guy was full of himself and she did not like his personality. So she did what she felt would humble him and she had some food on her feet which she had him lick. Her feet were smelly and sweaty and he had to lick food off of them.
When this mistress realized that her slave was being a bother, she called him to where she was resting and she forced him to lick her feet. She instructed him to suck her toes as well as lick her feet. He could not say no as he knew it would be an invitation for him to get an even worse punishment from the mistress. So he did things her way.
This mistress was tired of the kind of things she had seen her slave do and she knew that it was time for her to do something different if she was to have her slave change tact and do things the way he was supposed to do them. That is how and why the mistress cruelly used her dirty feet to trample him and inflict a lot of pain on his face.