When Ranma was young, he was taught a supposedly unbeatable martial arts technique: the neko-ken, or cat fist. Genma taught him the technique by wrapping Ranma up in fish sausage and throwing him in a pit full of starving cats over and over, until he snapped. The training was a success, from a certain point of view. However, it left Ranma with a terrible fear of cats, and he can only access the neko-ken by being frightened out of his mind, at which point his consciousness flees, leaving him with the instincts of a cat--a very powerful cat.
Many fanfics focus on the neko-ken, by trapping Ranma in the technique, attempting to cure his fear, or having characters berate Genma for teaching him the technique.