Mistress BlackDiamoond is good at nipple torture and she wanted her friends to be too. That is because it is an effective means of punishing and teaching a lesson and could be applied in any scenario. So she had it used the way she wanted and the mistresses had a great time teasing the loser, torturing him and learning from their friend. The guy they practiced on was paid for his troubles as he had done nothing to warrant the punishment and torture he got.