But How

I know when you go to the FarPlain that is when your sword is equiped with the waterstrike ability. But how does Chappu spirt go into the sword or is it somthing unknown if you know tell me

Pyreflies. Everything in Spira happens via pyreflies. (no seriously....)

At that point, Chappu is nothing more than a bunch of pyreflies. You see pyreflies show visions of the past in the game, you know that pyreflies make spheres. Pyreflies do a lot of stuff, and the pyreflies that were once Chappu probably did it (even though you don't actually "see" any pyreflies doing anything).

It's because your signature is massive

Haha. Um...probaby because of the pryeflies and because Square made it that way.

Perhaps it also uses the same principle as powering up the celestial weapons? I don't see any pyreflies there, but something magical happens.

The Celestial Mirror and the big rock at Macalania powers it.

Well I know that :p
I mean the principle behind it. There is a strange force that empowers the celestial weapons, so perhaps another strange force, and not pyreflies, empowers Brotherhood.

EDIT: My "m" key sucks

It gets waterstrike when you get rikku and the customize ability.

Pyreflies Emulate the Fayth or the Fiend.

final FANTASY. fantasy meaning magic and magic meaning something crazy and impossible porbbably in this case involving pyreflies but seems as this is final fantasy the answer to most of your questions is "magic".

there's a pyrefly stuck in tidus' sleve. it made the sword powerful so tidus would move his arm so that he could see the sword better. then it escapeed

its because they needed a way to make the sword useful, and this was the frist thing they could think of.

i hav a quick question. if u put another ability brotherhood with customize before u get to that scene with chappu, can u stil get the abilities?

You can't customize brotherhood.

exactly, besides, you dont have very good ability chooses.

Think about it, it gets waterstrike before you go to the THUNDER plains... having waterstrike would make the enemies easier... although the only enemies that Tidus fights there are pretty immune to magicks...