If I narrow my topic for an Essay by narrowing my topic to a single spoke or a sub-header on a linear map, how does this type of prewriting activity prepare me for outlining?

I love the brainstorming sessions - it is a very creative time and fun to do. In brainstorming, no idea is rejected - they all get written down, as they will be assessed later. You can set a time limit for brainstorming, say 15 minutes, or whatever you want to allow. In this part of the process, no idea is too silly or stupid, none of this "tried this before and it cannot be done" stuff. All ideas and thoughts are welcome.

So in this time, you have gathered all the ideas, then you want to go through them and sort. What is relative to the subject, what is not. What fits in the scope and what is not going to fit. Cite your reasons, so that when you review, your notes are right there ( not that you have to write those reasons down on that particular paper, but when you jot the idea on the notecard - good place to put the rationale).

It becomes a process of clarification, justification, and elimination. So you go from, let's say 15 items down to your workable amount that you find is needed.

From there, you can go to your outline and from there, to your draft, and then to your final presentation. You have great sense of accomplishment when it is all said and done.

Good luck to you!