In a femdom story, a chastity belt can symbolize power and control. The woman in the dominant role uses it to control the man's sexual desires, showing her authority over him.
A chastity belt in a femdom story can symbolize protection as well. It may seem counter - intuitive, but from the femdom's perspective, it could be protecting the man from his own baser instincts. By restricting his sexual access, she is guiding him towards a more refined and respectful way of interacting with others. It also symbolizes the femdom's ownership of the man in a sense, as she has the key to his 'liberation' and can decide when and if he is ready to be free of this restraint, which is all about power dynamics within the relationship.