The Evothaum I intended to work basically like evocation, only the initial cost (calling up the power) is free as you're drawing it from the rune directly instead of from yourself, and you must already have Evocation as a power to use it. On the flip side, you don't need to be a runecrafter to use the power of a rune in this manner.
So, say you have a 6-shift Fireball rune, and Harry Dresden wants to turn it into a spell. He takes the rune, and draws the power from it--no effort on his part to generate the power, so he's got a 6-shift spell called up, no mental stress. But, he still has to roll to target and control it. So going from his base 3 Discipline, he has to either make the roll, take backlash, or let some go as fallout. Harry can shape the spell energy however he wants, but, it has to A. remain an attack, and B. remain a Fire-based spell.