He does her tits

Tom got some duct tape and poked a couple dozen thumbtacks into them. then he put another layer of duct tape over the backs of the thumbtacks, keeping them from being pushed out. He took Sarah's bra and lined the inside of each cup, and the part in between, with the thumbtack-duct tape layer. He ordered Sarah to put it on and, of course, she did so.

The tips of the thumbtacks rested gently against Sarah's breasts and the skin between them. Tom spun her around so she was facing away from him. He put his hands over her bra and pressed it slowly into her, holding her tight against his chest. Sarah squeaked, then made sputtering noises as if to question what was happening, and then started saying "ouch ouch ouch" over and over again. Her breathing became more ragged and she shuddered, but made no effort to protect her skin from the thumbtacks, which were now penetrating her skin and drawing blood.

"Sarah," Tom said, "put your hand right between your breasts and push your bra into your chest." Sarah obeyed without hesitation and pushed the thumbtacks in her bra straight into the skin over her breastbone, causing them to break the skin and stop only when they reached the bone. At that, Sarah climaxed and fell to her knees. With tears in her eyes, she looked up at Tom and said, "Thank you, thank you for doing this to me. I really can't say enough thank-yous for this wonderful gift of changing me into a submissive masochist. What will you do me next?"