Flash as the Main Light Source
Fill-in flash is generally flash that fills in shadows and reduces contrast from the main light, but you don't always have to use your flash this way. In fact, one of the best uses of flash is to underexpose the ambient light so that your flash output becomes the main light source. When you work this way, it's as if the ambient light is acting as the fill light to reduce the contrast from the main light, which is the flash. When using your flash as a main light source you want the flash to create shadows for an impression of depth and three-dimensionality. That can mean taking the flash off your camera. In order to use your flash as a main light source, it's important that you accurately understand the intensity of the ambient light, this is so important for portrait or wedding photography. In our picture of this lovely couple at a weeding in Somerset, the ambient light gave an exposure of 1/200 sec at f/2.8 on a setting of ISO 100. This combination was carefully chosen. The ISO was 100 as that's the camera's base setting for the best quality: a shutter speed of 1/200 sec to reliably sync. This combination gave an aperture of f/2.8 for the "correct" exposure and to make the flash the main light source, we needed to reduce the effectiveness of the ambient light, which was very low due to the candles light and the light coming in from the ends of the corridor.
This means underexposing it, though by how much depends on the precise situation and the results you are looking for. A couple of stops is usually a good amount to start with. To do this, either take control manually or set exposure compensation if you are using an auto mode like Shutter Priority. To underexpose the ambient light by two stops I could drop my ISO by two stops, but as the camera was at its base, this wasn't an option. You could also increase your shutter speed by two stops, but as this would take the speed up above the regular flash sync speed you could then run into serious issues. Thus the only option is to close down the aperture by two stops. If you are using an auto mode with exposure compensation, it is best to stick to Shutter Priority mode. Using Aperture Priority mode could see the camera setting too high a shutter speed for your sync. If you are using a dedicated wireless system like CLS or E-TTL, your camera will avoid errors, but if you're using conventional radio triggers, sync speed is a real issue, so stick to Tv, for my shot, I stuck to manual mode and actually underexposed by 2 1/3 stops, reducing the aperture from 1/2.8 to f/6.3. I then introduced a flash as the main light, the flash was controlled manually, and the power adjusted until it correctly exposed the most important part of the shot. In this case, the low light meant the power of the flash was easily sufficient to overpower the ambient light, especially at such short distances. On a very bright day, and if the flash was used through a power-sapping soft box, for example, the flash might not have been bright enough.
With wedding Photography on very bright days you either have to somehow bring more flash light in by using a more powerful unit, reducing the flash-to-subject distance, removing light modifiers like soft boxes, or moving your subject into an area of shade.