The slave is trapped in the floor with a hole for his face, he cannot move or turn away, a perffect victim for her feet.
Careful in the beginning she kneads the face with her feet, steps lightly on it, shows him how much pressure will come.
Soon she gets more confident and begins to walk and step with all her weight on the face.
How's about a little dance.... dance on the face... 60 kg pressure for kneading and face-deforming.