Lady Shay wanted to scare her slave shitless so she did it with her shrinking threat. The mistress described what she was willing to do to him with her sneakers to teach him a lesson he would never forget. The mistress could tell that her threat was working and she was satisfied that the slave would never disobey her or do anything she did not approve of because he feared what she would do to him.
Lady Shay felt that only trampling this guy would change anything. She had tried to talk to him but it did not seem to help. So she had to come up with an alternative way of doing things and that is why she painfully trampled him. To make it even worse than it already was, she trampled him with dirty sneakers. He had to lick them too before she was done with him.