This is when the final Boss Battle is spent with some sort of Power-Up or item or equipment not found anywhere else in the game. Presumably, it's the only way that boss can be defeated (possibly indicated by a Hopeless Boss Fight beforehand). The probable reason it's only available in this fight is that it would severely screw up game balance if it was available anywhere else. Purposefully Overpowered usually ensues. Developers should exercise caution when using this trope. Done well, it can make for an exhilarating experience. Done poorly, it can feel like a massive cop-out.