Character chooses a rather long safeword (e.g. Supercalifragilisticexpialidocious, if not longer) . Dom prevents use of word by forcing the character to interupt it with moans and cries via some mix of pleasure and pain. Ideally, this story should be consensual. Thus, the inability of the sub to use the safeword should be either about play or breaking limits.
In the case of play the sub really isn't able to use the word but honestly doesn't want to. The dom is just playing and the idea is to provide an extreme amount of sensations for the enjoyment of the sub. The measurement of success is how poorly the sub is at saying the safeword. In this case, perhaps partway through the sub should succeed at saying the safeword, the dom should stop, and the sub should complain and say to keep going.
Alternatively, the story is about pushing limits. The sub does want to say the safeword...but also doesn't. I think the first idea is more fun.
In the case of play the sub really isn't able to use the word but honestly doesn't want to. The dom is just playing and the idea is to provide an extreme amount of sensations for the enjoyment of the sub. The measurement of success is how poorly the sub is at saying the safeword. In this case, perhaps partway through the sub should succeed at saying the safeword, the dom should stop, and the sub should complain and say to keep going.
Alternatively, the story is about pushing limits. The sub does want to say the safeword...but also doesn't. I think the first idea is more fun.