I am sorry but i do not understand one's concern about this difficulty name, here is a quote from the ranking criterias :
Correct me if i am mistaken but i am almost sure to be reading there that "marathons maps" are free to use any sort of difficulty name, may it not be related to the song or anything. See below the requirement for a map to be called a "marathon" :
The play time of the map being over 6 minutes which itselft is over 5 minutes, i can safely assume that this map is actually a "marathon".
From there i supposedly do not need to provide any explanation about this difficulty name as it is perfectly rankable and does not break any rules. Now if there were to be some more "hidden" rules like "the difficulty name must be approved by the great lord and saviour of taiko also called OnosakiHito", i request them to be added!
However i will still try to give you guys some deeper explanations as it seems like this subject really does concern you :
First, the letters i chosed to use in the difficulty name are in fact taken from the name of the song itself! If you remove the right spaces you may be able to read it . . . Brain Power! there lies a big link to the song.
Second, let me take a quote from a well known part of the lyrics from the song :
There i highlighted specific parts where we have uppercase letters AND spaces between them, now you may be able to see throught my reasoning.
The last thing to talk about is the theoric aspect of this difficulty name. The "power" (brain's one in this case) usualy allows you to EXTEND your capacities/abalities. So basically the uppercase letters are there to represent the
high state that the power puts you in while the spaces are there to represent this feeling of
extension that the power provides.
Now if you guys are still frustrated because you believe that the difficulty name is as it is just for "fun" or anything related to that, i will gladely invite you to evacuate all your stress throught the star rating system!