The game data is encrypted, with various keys along the ROM. There is a custom chip on the ROM that permforms the decoding while the game is running, but when you rip the ROM, you get the encrypted data.
Brute force means that you try every possible key untill you find the good one(s). It requires a lot of processing time. This has to be done one time only.
In this peculiar case, you get a table with all the keys. You see the result of this in the pif2.dat file located in the main pj64 1.5 folder.
"Brute Force" doesn't require any hard work on the emulator side, the hard work has been done by Azimer's and Tooie's computers.
(Tooie is PJ64 1.5 lead developper, maybe another emu author in disguise? Who knows.. Do you remember CoolDeadalus? Whatever, it's so easy to be anonymous on the web :shifty: )
Once you have the table, the emulator has to apply the right key to the right part of the ROM.
Non brute force would mean that you'd have to find the algorithm used in the chip to choose the encryption key for a given part of the ROM. Applying the algorithm on the fly would probably be slower than using the table described above, so you'd have to generate it anyway...