This seems to cap at least Melee, Ranged, Fire, Etheric (probably also Stealth) Power to +0. If power is less than zero, it remains diminished.
Bonus Power when the Defender has no Technique and Evasion Power remain buffed.
I guessed maybe Fire Power would cap the other stats at that but it doesn't seem to help. EDIT: Fire Defense works! With 5 FD, all my MP, RP, etc are capped at 5.