When this mistress felt that she needed to send a cruel message to this guy, she did it with her fisting fetish. The mistress felt that the best way for her to do what she wanted was to use pain and that is why the fisting idea came to her. It worked as she expected it to and she had a great time doing it. And the message was home.
Lady Lucy needed to fist this girl because she was insatiable. She did not like how this girl did things and how she acted. So she had to punish her for it and that is why she chose to fist her. That way, she would get the satisfaction that she had been chasing for a long time. The girl did not know it but she was surprised to be satisfied by what she did.