You can invoke aspects to add +2 to any roll, so long as the table agrees that the aspect relates to that roll. So, for example, I might use the 'Arcane Senses' from The Sight to use Lore for an Assessment. I determine that there is the aspect "A River of Magic Runs Through It" on the scene. Shortly thereafter, I encounter a bad guy I want to torch. I tag the aforementioned aspect for a +2 to my Discipline roll when I cast the spell, which makes it hit harder. I don't think you can boost the draw power part of Evocation, but adding to the roll helps with both control and attack roll, which translates in turn into damage.
The quote in Changes (and there's no spolier included here) is "A brief halt, true, something that lasted no more than a handful of seconds -- but it was time enough for me to reach down to touch the slow, terrible power of the ley line flowing beneath my feet".
By the way, I recall another instance that he did something like this. I think it was against a Frog Demon or some such, and he drew power from a storm to blast the demon. Storm Front seems likely to be the source for that one.
