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ETTL-II and Exposure again.

Hi everyone, I'm a new user.
ok, I'm a beginner, and my question might sound stupid to someone, but there is something about how ETTL-II (Canon metering system) really works that I need to understand in order to dispel
my doubt, take the next step and move on finally. For the record, I've already read a few articles about this subject on this site and elsewhere on the internet, but getting a complete understanding of how Ettl-II works is something that is driving me crazy.

I read the text below on this site,
"With TTL flash, your camera and flash work together to control /calculate flash exposure ..
and then adjust the power the flash emits, to give you (what it deems to be) correct exposure …
regardless of your choice of aperture, or your choice of ISO, or the distance of your flash to the subject. 
Of course, these things have to be within reason.  You have to work within the capabilities of your flash."

But how does [a correct] ambient exposure metering affect TTL- ETTL flash?
For istance, if I choose spot-metering or something and take a correct exposure on my subject by adjusting aperture, shutter speed and ISO, neither under nor over-exposed (O, what my camera's metering system deems to be), how and why the flash correct exposure is "more correct" of what my ambient exposure metering says? I mean, it makes sense to me only if I underexpose the scene on purpose.

I'm not a native speaker of English, and because of that maybe I failed to get my point across, sorry for that.

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