User blog comment:Zolnir/RWBY Episode 8/General Reviews & Speculations/@comment-10390252-20130913113025

I think, to use your RPG anology, aura effects are 'cast from HP' or from Fatigue. So, after his show against the two King Taijiitu, plus running for who knows how long to keep up with Nora and her 'mount', he'd probably used up most of his energy reserves.

FWIW, I suspect that aura-accelerated fatigue induced by heavy aura use is going to be a key weakness in all the characters but particularly for Ren, who seems to rely more on unarmed techniques than his, frankly, decidedly under-powered weapons.