It all seems complicated until you realize that there are two metering systems, the one in the camera that in Matrix, considers the entire scene with emphasis on the area of the selected focal point. The second independent metering is in the flash. When you meter there is no illumination other than the ambient to that is reflected in the readout. Taking the photo without flash will be exposed normally, based on the ambient light. So if there is enough light without flash, and the shutter and aperture are set for the reading, turning on the flash will only add light above the level the meter reported and result in overexposure. If you metered and adjusted the exposure settings to some underexposure level, say 3 stops under exposed, the TTL flash will make up the difference, adding that 3 stops. So for normal non-backlit scene, you can control the relative scene to subject exposure by adjusting the camera exposure for the degree of scene or background exposure you seek and let the TTL flash determine the subject exposure. Underexposing more according to the meter means the background will be underexposed by the same amount, darkened, below the exposure level calculated by the flash for the subject. Setting the exposure of the overall scene close to, but lower than normal exposure will mean the background and subject will be close to each other in illumination. There are many scenes that the subject is reflecting less light than the ambient, at night or day, almost any ambient conditions can have the scene brighter than the subject due to position, a shadow on the subject etc. That is a case for TTL BL Balanced mode on the flash. Ambient is still metered as before for the whole scene and if the shot is taken the scene would be properly exposed, but the subject is darker, a typical backlight situation, for example the subject at the beach at sundown. Taking that shot without flash means well exposed beach and setting sun but a dark subject whose back is to the source of light. Exposing for the scene is handled by the camera meter as normally the case but the TTL BL metering of the flash, the flash tries to illuminate the subject to equal the scene illumination. The two metering systems treat the scene and subject independently with the flash concerned with the subject.
Flash in a low lit scene is easier than most other exposure means using Manual. Set the camera exposure for the optimum for DOF of your preference, and shutter speed based on knowing the fast pulse of light will freeze motion(but is subject to ghosting if ambient it higher) and let the Flash calculate the correct light level to expose the subject properly. A dark restaurant or party for example could be set in Manual, 1/80 ISO200 and f/4.0 and leave it that way all night, letting the flash in TTL calculate and properly expose the subjects.
If the lights come on and metering shows good exposure is possible you have two choices, decrease exposure by the degree you want to bias the exposure to the subject instead of background, or turn off the flash. When using flash, turn off Auto-ISO and use manual whenever possible. It all makes sense once you think of there being two independent metering systems with two different focuses of attention.
If there is enough light to get proper exposure according to the meter, using any flash at all will add to the already suitable ambient lighting so overexposure will result. The exception is if the subject is darker than ambient, then switch the flash to TT BL and expose the scene normally as if the subject was not darker, allowing the flash to fill in and balance background and subject illumination.
Note that some strong backlight and very dark subject can't be balanced by a flash. The strong sunset light versus a subject far from the flash is an example where BL is not going to generate enough light. Studio strobes would be used to try to "overpower" the sun. For flash photography you can just move closer to the subject to greatly increase the illumination of the subject.
Once these points are second nature then you can start considering creative control of background versus subject by calculating the degree of under exposure or flash fall off to get very dark background even in a normally lit room. Flash becomes a lot of fun once some basics are really understood. Stan St Petersburg Russia