In the beginning Isabella steps on a scale for knowinng how much weight will destroy the face of her slave.
Then she steps on his chest and soon she has her foot above his face, that he can see what will crush him.
Slowly she teases and plays with the face under the sole, the it's time for real fullweight facecrush.
The sole of her sneaker is quit hard and squeezes his nose and mouth, and remember Isabella isn't a lightweight (you will see the exact weight in the clip)
To the end she steps on the camerafor a point of view (POV), you will see the same as the slave did under pressure and pain.