Mistress Iside was interested in testing this guy's pain threshold. This she did by slapping the shit out of him. The mistress knew that the guy had never been as slapped as she slapped him and that was why she was interested in torturing the guy and seeing his reaction to what she did as it would communicate a lot to her than he would ever tell her if she asked.
Mistress Iside and her friend wanted this guy to get the punishment of a lifetime. He was tied up and rendered helpless. Then the mistresses spat on his face and covered it with their spit. He could not wipe it off his face and as a result, could not even open his eyes. And that was the perfect way and time to slap the shit out of him as he could not see the slaps coming.