Related questions matching - How does it work?

Question

In admin interface, there is an option "Related questions matching:" choices include Narrowest to widest. How does that work. Honestly, there were no changes in questions suggested as I changed the levels.

Specifically, my question is for an issue I am facing:

Let us say, there is a question "How do I travel from London to Manchester" (let me also state tags London and Manchester have at least ten questions tagged "london" or "manchester" )

Now let us say there is another question "How do I travel from Boston to Washington"

Q2A currently suggests "How do I travel from Boston to Washington" as the first related question to " How do I travel from London to Manchester" across all choices - from widest to narrowest.

I am using only tags for classification and have set q2A to suggest 8 related questions

It will be great if Q2A suggests questions that are from pool of questions under the tag of "london" /"manchester" rather than "How do I travel from Boston to Washington". How can I achieve that?

Is there a way for me to specify some words in question titles (like bus/hotel etc) that related widget needs to ignore while suggesting questions.

Thanking you in advance.

Comments

I will let gidgreen answer in more detail, but generally it gives priority to more "unique" words in the questions. So it should bring up other questions with London it, instead of matching common words like "how do i". But if you don't have many questions then it's difficult to match stuff.