When Ranma was ten/six years old (manga vs. anime), Genma tried to teach him the Cat Fist. He tied fish sausages around Ranma and tossed him into a pit of hungry cats, presumably with the belief that this would imprint the fighting movements and instincts of cats into Ranma's mind. However Genma had made the critical mistake of not fully reading through the description of the technique, which stated that nobody should try and train a person in the Nekoken through these methods. He tried to cure Ranma by repeating the process with dried sardines, salted sardines, and fish cakes, but these attempts had the opposite effect and only exacerbated Ranma's fear even more. Despite this, neither have sought or attempted to cure Ranma's phobia since, and it is never mentioned how to actually train someone in the technique. Assuming that there is a "proper" method and this is not merely one of the oddities of martial arts histories; whilst the manga has the "next page" stating that only an idiot would try to teach someone the Nekoken, the anime has it declare that the move has been banned for the severe psychological distress it forces on the trainee.
Due to the trauma, Ranma possesses an intense fear of cats. It bears mentioning that, contrary to fanon, Ranma's fear manifests itself proportionally to his proximity to an actual cat. He does not get nervous when the word is used, nor is he unable to say it, and he can be around cats if they are a sufficient distance away from him. However, when a cat draws too close or he hears a meow, he will visibly start. If forced into close proximity with a cat (or suddenly ambushed by someone wearing a cat mask and meowing), Ranma will physically recoil, and do his best to retreat from the source of his discomfort. If Ranma is forced into prolonged exposure to a cat, his panic will ultimately build to the extent that he will become semi-catatonic, switching over to a secondary personality with a cat-like mentality.
When Ranma falls into his cat-like state, he starts acting like a cat, apparently using his power more efficiently by fighting like a ferocious animal, with swipes of his "paws" which have torn tree-trunks to shreds from a few meters distance. This does not seem to be the result of chi-blades, since no visual effect has implied it while they were clearly distinguishable when generated by Herb. Rather, Ranma's "claws" seem to be powerful air pressure swipes, somewhat similar to those of Tatewaki Kuno or Ryu Kumon, but with far less shown destructive power than the latter. While in this condition, he even managed to disperse Cologne's Shark-Fist water blast, though it is uncertain how much she held back, or to tear effortlessly through the ice boulders wielded by her Fist of the Ice Bear technique.
However, the mental influence of this technique goes beyond merely giving Ranma enhanced fighting instincts. Ranma's "cat mode" is so powerful that, mentally, Ranma truly becomes a cat, driven by instinct and suffering severe impairment to both intellect and attention span. This is the great weakness of the technique, making Ranma vulnerable to both catnip (which will make him "drunk") and distraction (throwing something and saying "Fetch!" has proved enough to get rid of him), as well as being completely incapable of strategizing or using any level of higher thought. This makes the technique virtually useless, as Ranma in cat-mode is completely unpredictable; there is no way of controlling who he attacks or how badly, and he is just as likely to get bored or scared and run away from a fight as he is to stand his ground.
Ranma cannot remember what he does when in his cat-like state. He has been brought out of the Nekoken by being doused with water, or by falling asleep in the lap of someone he trusts. This used to be an old woman who lived in the Saotomes' neighborhood, but is currently Akane Tendo.
Also worth noting is that Ranma and Akane shared their first mutual "kiss", while he was in this state, as he impulsively rubbed his face against hers, and it was believed to be a kiss by everyone watching. Although Ranma did not recall doing this, Hikaru Gosunkugi later showed him photographic evidence. This was not Ranma's first kiss however, which he received from Mikado Sanzenin while in his female form, to his tremendous embarrassment.
Activating the Nekoken is neither a swift nor voluntary process. Particularly if he does not want to, Ranma can resist the urge to fall into the Nekoken for a substantial period of time. Even if he does actively want to engage the Nekoken, it is not an instantaneous process to activate it. These further showcase why the technique is considered flawed, and held up as a canonical weakness of Ranma's rather than a strength.
Times Used in the Series
The Nekoken has only been used a handful of times in the entire series of Ranma 1/2.
The first time that Ranma used it was when Tatewaki Kuno attempted to discover Ranma's weakness with the aid of either Hikaru Gosunkugi (manga) or Sasuke Sarugakure (anime). It took being trapped in a small space with a hungry tiger whilst multiple cats clung to Ranma for safety before he succumbed to the Nekoken.
The second time that Ranma used it was in the final battle of the Full-Body Cat's Tongue arc. Desperate for an advantage over Cologne, Ranma persuaded Shampoo to help him/her, and then jumped into a nearby source of water. As the confused girl-turned-cat clung to Ranma's head, Ranma ran around screaming for several minutes, with Soun Tendo and Genma Saotome taking it in turns to force Shampoo to cling to Ranma's face and verbally encourage him/her to give in to the terror s/he was feeling. Finally, Ranma succumbed and entered the Nekoken, in which s/he was unpredictable enough that Cologne conceded victory.
The third time that Ranma used it was during the debut of Maomolin. Whilst Ranma was terrified throughout his encounters with the enormous bakenko, it wasn't until Ranma was turned into a female and embraced by Maomolin, who proceeded to lick "her" in the face, that s/he snapped and entered the Nekoken.