The slave was prepared for his upcoming duty before, when Miss Cherie and Lady Jane had beaten him brutally. After they successfully broke his will, he decided that he had to swallow his disgust and deal with his fate. And his fate will be brown, nasty shit from the girls. First they use him as a waste pipe that's under their toilet chair when they piss and shit into his mouth, and then they feed their shit with a spoon to the slave.