This slave was made to lick this mistress's feet because she had angered the mistress badly. And as she licked her feet, she had to humiliate the girl further and she did this by forcing her to smell her farts. The mistress told her to enjoy the farts and it was tricky for her but she had no choice but to do it as there was no alternative to that.
This mistress loves honesty and she was not going to allow her boyfriend to not practice it. She had to make him be honest with her at all times. That is why she went out of her way to slap the shit out of him when he lied to her. She even trampled his face with her feet before she felt that it was enough punishment for his silly ass.