I warm up on my slave's balls with some barefoot trampling, pressing his balls flat in various ways seeing how much pressure I can put them under. Then I switch to my heels and stand on both of his balls to get them nice and swollen for when I put them in between my heel and shoe! His right ball managed to escape for a second but I put it right in it's place and pressed down on it, pancaking his swollen ball around my whole heel as his balls emptied under the pressure!