Wow. Amazing training here. Arms in a reverse prayer, yet forcing her to keep perfect posture with a heavy object on her head. If it slips, those nipples get ripped up. On top of it all, a hellish chastity to keep her in her place. Uncomfortable bondage reminds the sluts of their constant bondage to their Masters. Their constant discomfort reminds them of their bodily sacrifice for Masters pleasure and entertainment.
Such a creative way for a task that requires full concentration.