This mistress could tell that this girl did not know how to give good head. And she humiliated her for fun and also to do her a favor and teach her to do it. So she used her feet to teach her. The girl had to lick the mistress' face and even smell her stinky socks when she did not do it the way she instructed her. And the same was repeated until she did it right. Then the mistress made her pay her for the training.
This mistress belittling of any kind and when this guy belittled her, she had to show him her true colors. She did not hesitate to do it as she was sure that it would mean he never did it to her or anyone else. So she crushed his neck and his chest with her bare feet and made him pee himself in the process. The pain spoke to him more than her words.