The slave is trapped in the floor with a hole for his face, he cannot move away, he's at her mercy.
In the beginning, she places the nasty profile of her boots on his face and shows him what she could do with them... maybe meatloaf?
But she's not a monster, shows some mercy, sits down on his face and removes the boots.
Now he has to inhale the odor of her socks, later of her bare feet.
To the end she steps fullweight on him, so he can feel the pressure on his face.