I tend to thinko f it as manipulating the numbers as a primary focus of the character.
Imagine two people playing a monk.
Person one develops a monastic tradition and chooses apprpriate feats. to the tradition. They figure out their stats in order to make themselves good at the two or three thematically-tied focuses of their school.
Person two decides they want to be really good at stunning blows. They search through four different sourcebooks to find four different feats that either increase the DCs of stunning blows, or give extra funky powers to their stunning blows. They find two items in two different sourcebooks that combined increase the stunning blow DC. In the end, their character can't do much of interest besides the stunning blow, but their stunning blows have a DC in the high twenties or low thirties and paralyze opponents for several rounds, making each attack they perform successfully an almost unstoppable deathblow.
The latter player is doing unacceptable minmaxing, from my perspective; the first one isn't.
Daniel