Heat

Lock the girl in a cage and make her sweat. A lot. Put an electric heater under her. Chain her head, wrists, ankles and waist so she could barely move. Make her suffer. Until there was begging and crying. Preferably both. I got my wish, although she stuck it out for a long time before the harness gag got her. The sweat dripping down her face caused the panel of the gag covering her mouth to seal off her air completely. I had to let her out. But not for long. As soon as she caught her breath I was waiting for her.