-FEC turns the flash down and +FEC turns the flash up. so to sum up (+ adds light) and ( - takes light away). So if for instance you are taking a picture of a dirty coal miner at the entrance of a dark mine your camera/flash will try to make this picture MID grey (18% grey) Now 18% grey throws most people a bit, basically it means half way between pure black and pure white ,with neither showing any detail. Now if we assume that the entrance to the mine is pitch black then your camera is trying to make it 18% grey, which would make the image look all washed out and over exposed.
This means you would dial in -FEC of say 1 1/2 to 2 stops. The same would go if you don't use flash. You would reduce the exposure by either adjusting the shutter or aperature to reduce the light.
Now if for instance you were to take a photograph of a leggy blond in a white swimsuit on a white sandy beach, the camera would again try to make this 18% grey by taking some light away from the subject. Now I'm not sure about the rest of the people here but a leggy blond in a white swimsuit looking all grey and dark does nothing for me.
So in this case you would add some light by setting it to +FEC of say 1 to 1/2 stops
And if not using flash you would open up the aperature or change the shutter from say 250th to 125th
Having said all this please keep in mind that FEC is for the flash only. So when you set your cameras exposure at say 125th at f16 for a sunny day at 100 iso (standard base exposure for sunny days out in full sun) you can then play with the FEC to fill in the shadows of your subject. This will get rid of those horrible dark shadows under the nose etc. The problem with fill flash is that it doesn't always look that great or natural. If your subject looks like it's in full sun even though you have the sun behind them you might dial in -FEC so that it looks more natural. She same goes if the subject is still too dark and looks more like a silhouette then you might increase the FEC to +FEC
FEC is very subjective. What you like is not what the next person likes.
As the old saying goes "How many photographers does it take to change a lightbulb? Twentyone. One to change the bulb and twenty to tell you they could have done it better."
