As with any fantasy, there is always a risk. The risk that someone will find out her secret desires and embarrass her. The risk that she could be hurt by her partner. A special trust is required before you can open up and share you fantasies with someone. And then there is the risk of tying yourself up, and not being able to get free. So she had invented a "safety person". Her master would save her if something went wrong with her bondage, her self-bondage. If she was unable to escape after her orgasmic fantasy, her master would help here get free.