Mistress Mystique felt that her slave was getting out of control and she needed to control him. She was not going to sit back and watch him spiral out of control and misbehave. So she used her big ass to punish him. She did it by smothering him and choking him with the big butt which he had also had to lick as well as smell before she let him go.
Mistress Mystique does not let anyone mess with her especially when she knows what she wants and how she wants it. When the mistress realized that she was dealing with a disobedient slave, she had to make him change. And to do this, she used her facesitting fetish to torture him. The punishment worked and he had to do things her way. She slept a happy woman as she knew how to make people change.