Because this mistress was out to make sure her slave never did what he had done again, she chose to use her boots to do it. It had to be the sort of punishment he would not forget and the mistress made sure that he would never forget it as long as he lived. That is why the mistress cruelly trampled his fingers with her boots and made him pee his pants.
Mistress Sandy wanted her slave to stop his laziness and that is why she opted to use her boots to trample and stomp on his hands. If he did not know how to use his hands to make a living, she was going to ensure that he felt a lot of pain on his hands everyday until he changed. When he could not take it anymore, he chose to do what she wanted.