The beastmen have a proverb called 'Ors atrzz ma sok,' meaning that even an animal at the end of its rope has the strength to make one last effort. In Chinese, it would be akin to saying that a cornered rabbit will bite back. Brand felt this phrase perfectly described his current situation, though only the first half was relevant; the difference being that he wanted to bite but lacked the strength.
The Platinum War Steed still had some energy, but Brand himself was out of it.