Mistress Vivienne is riding a slave’s face. It’s such a good method of reminding slaves who is boss - buried under her ass and unable to draw breath. The slave desperately attempts to suck in some air. Mistress laughs - You’re being smothered under my big butt, it’s flattening your face and denying you breath. Now Get your face under my queening stool”. Ah much better - far more comfortable for Mistress and just as effective.