Transcranial magnetic stimulation alters the function of specific brain circuits, mostly brain circuits related to the regulation of emotion. People who get a good response say that they feel more normal inside, that they feel like they have a better, more consistent experience of their internal emotional life. They don't need to do things like binge and purge anymore, or if they have PTSD, they don't get so activated and cut themselves and stuff like that.
I wouldn't exactly call it retraining. It's more like restoring normal function to brain circuits that have been demonstrated to be not functioning normally. It's the same for deep brain stimulation.