A rare-ability is a physical constitution that is not necessarily a genetic mutation. Meaning an ability that can be obtained or learned, or you were born with that doesn't necessarily pass through blood. For instance, Deidara is a blast-element user, someone who can manipulate explosive force through elemental manipulation. In this sense he doesn't have a rare-ability. However, by obtaining a forbidden Iwa Kinjutsu, Deidara was able to manipulate his chakra into clay substance. In order to manipulate clay substance, the Kinjutsu gifted Deidara with mouths on his hands and chest. With this his rare-ability explains his "hand-mouths" and it makes sense as to why he was referred as a blast-element user but still has a uniqueness to him in his artistic ninjutsu.

Usually rare-ability is posted up with your application. Most people with other powers will not receive a rare ability for the sake of fairness. For instance, you wouldn't be able to have a lava-element bloodline and then obtain Earth Grudge Fear. Rare abilities could possibly be obtained under some circumstances. 1) The character application is normal and has no previous abilities at all. They can obtain a rare ability to increase their power. 2) The rare ability is passive and isn't offensively used. It simply just changes the way in which things can be done. For instance, Deidara is a great example of this. 3)The rare-ability acts in conjunction as an extension of power that a shinobi already has. For instance, Enton would be a rare ability if you were able to manipulate Amaterasu extensively. 4) Other various methods and situations that could be presented later in time. Please do not think you have an excuse to obtain a rare ability for every thing, these are normally reserved for people wishing to have a unique characteristic to their characters. Most people will opt for a bloodline and history or follow a cannon clan. If you are one of these types of people, you will probably not need to post here at all.

It depends on how it works, but most of the time you will have to train or sacrifice your time and effort to get a rare ability. In the most common cases of rare abilities, some type of training to gain it must be done. Deidara would have to perform a Kinjutsu for his and then learn how to use the new ability, Kakuzu would have to perform the Earth Grudge Fear on himself to use it, Sasuke had to use Amaterasu extensively to use Enton. In most cases though people usually have a rare ability at the start. Like Hidan and Jashinism. Training will be broken down on moderator judgement, depending on your ability.