When Erina has to punish her slave, she has the perfect way to do it. She calls him to her and orders him to take her shoes off. She didn't wear socks so her feet are stinky and sweaty. When she orders him to the floor, she puts a piece of tape over his mouth and binds his hands. She puts her chair on top of him and places her filthy feet in his face. She knows that it's suffocating. That's her point. That's why she does it as a punishment.