For Halle, this slave's face serves a dual purpose. Naturally it makes a good target to slap when is isn't behaving correctly, but it's also a place for her to stand on. To put both her feet side by side on his face, resting all her weight on it while he mumbles from underneath it all. It's kind of funny seeing his face all obscured by feet. I mean she knows he is down there somewhere, she just can't see him anymore on account of her feet. Fortunately Halle doesn't weigh very much so his cranium should be able to withstand the pressure. If not then oh well.