I think that's the surge writers tend to get when they really want to write a particular scenario that's been stuck in their heads...or fresh in their minds. That's the "Writer's Adrenaline" I get. One day I might be doing something, and suddenly get this urge to write a scene. I find it to be a good thing bad thing sort of deal. While I do get to write a part I've been itching to write, the less fun part arises when I have to piece it together with the story, and figure out the (less interesting) events that lead to it.
There's a lot of plotting involved and, most of the time, that scene I've been wanting to write gets thrown out—the end result being understanding my own story more in depth. I think every writer has to experience this "Adrenaline" when it comes to writing. Pardon the quotations. :)
In other words...
Trainer is in a battle. ( I might write a pages about a this particular battle because it interests me.)
Then I might decide that battle, for the sake of plot, is unnecessary, maybe even downright a nuisance. Although, thanks to this analysis, I would have understood my story more and led it into a better direction than a five page battle.