Mistress Beh felt that she had to teach this guy a lesson he had never been taught before. That is why she went out of her way to spit on him and force him to swallow all of it. In addition, he had to eat other dirt from her feet as she filed them. He was shocked by the treatment but he knew she was mad and he did not want to piss her off any further. So he complied with her orders.
This mistress turned her dirty sneakers into clean ones by getting the slave to lick them clean. The slave could not understand why she was doing that to him but she was not going to explain herself to him as he was an intelligent person and needed to get the hints that she had been sending to him. The slave had to run away after that as he did not want such humiliation again.