Feistel is a permutation. That means it's a 1:1 mapping between a 16-bit # to another 16-bit #.

You run Feistel for each number 0->65535 (corresponding to the "stage" of the FizzleFade) and out comes the pixel to redden at that stage. Since it's a 16-bit number, some values will fall outside of 320x240 resolution, and you ignore those.