How Flash Works

ISO

When using TTL, ISO will not affect your exposure.  A higher ISO will will more effective when you are further from your subject.

Aperture

When Shooting TTL, aperture will not really affect your exposure, but your flash will work harder at f/5.6 than it will at f/2.8.  This means it will take longer to recycle and your batteries will drain faster.  Aperture will mostly just affect the focal plane in terms of how your image looks.

Shutter Speed

Shutter speed doesn’t affect your exposure or the power of your flash at all.  Shutter speed does affect the amount of ambient light in the room already.

Flash Compensation

Your flash’s job is to throw light onto your subject so that it bounces back to the camera and allows for enough light to expose an image.  So the more light you need to put out, the shorter distance it will be able to travel to your subject.

So for example, at -3, your light output would not be as bright, but would travel a further difference.  At +3, your light would be brighter but cover a much shorter distance to your subject.