Mistress Blackcat knew that a scared slave was good for her and that is why she did not hesitate to scare this one with her boots. The mistress used her boots to kick him in the balls and when she was done, she used the same boots to humiliate him by instructing him how to lick them. And he did. That is how he learned to fear her and do everything as per her wishes.
When mistress Eva's husband gambled the rent, she knew they had entered a critical stage and that things were likely to go south unless she did something drastic,. She used her boots to kick him in the nuts after she had stripped him naked. That was not all as she also whipped his naked butt to make him realize that he was never going to gamble again. That was the last time.