There's a currently active thread on the Glamour1 forum where a technique called feathering the light is being discussed. The original poster provided an image, along with a lighting diagram, that illustrates his attempts to feather the light across a model's face.
Rather than write about that shooter's attempts to feather his light, I thought I'd simply write about feathering light in general.
If we assume the brightest area of a directional light source is at it's center (e.g., where the flashtube is positioned) we can also assume that, as you move away from that center and remaining in the same plane, the intensity of the light diminishes. In other words, the light begins to fall-off or feather or gradually decline in brightness as you move away from it's source. There are a variety of ways to feather the light including from right to left, top to bottom, diagonally across an image, or around a perimeter.
Mola beauty dishes, for example, are specifically designed to fall-off, or feather, a half-stop from the center of the dish to the outer perimeter of the dish. Obviously, as you move even further away from the outer perimeter of the dish, the light will fall-off even more dramatically.
Using a Fresnel lens and focusing it in various ways is another way to feather, or produce a graduated fall-off around a permeter in a light source's appearance in an image.
When you feather the light across a subject you'll notice a fall-off (or diminshed exposure) from the brightest point made by the light to the dimmest. The more gradually the light falls-off the more aesthetically pleasing it generally appears. The more pronounced the feathering is, the more dramatic the image will often seem. Feathering is another technique which belongs under the broader heading of Light Control.
If you're using a light source that is not designed to produce fall-off, like a Mola beauty dish or Fresnel lens, you can still feather the light by angling it away from the subject. The degree to which the light is feathered will depend on the angle it is turned away from the subject as well as the distance between the light source and the subject. Since a light source placed further from the subject will produce light with harsher or more pronounced shadows and, conversely, the closer the light source the softer it appears, the elements of angle and distance work in concert to produce more or less feathering.
If you're using modeling lights, you should be able to see the feathering effect on the subject when you play around with varying degrees of angle coupled with various distances between the light source and the subject. If you're shooting digital, as most of you probably are, a quick review of the images will tell you how pronounced the feathering effect becomes as you adjust the position and direction of the light.
Large softboxes are sometimes difficult to use when attempting to produce a noticeable feathering effect. Softboxes are usually designed to minimize brightness in the center (less center hot spot) by using an internal baffle. They are also shaped to disperse the light evenly over a larger surface. In other words, the larger the source the more difficult it will be to produce a feathered effect across a small target area like a model's face. A smaller source, therefore, will produce more noticeable feathering. I sometimes use a a bare bulb with a small reflector and with a small grid affixed in front of it to produce more obvious fall-off across a small target area. By angling this light source and adjusting the distance to the subject, I can see the amount of feathering its producing.
The pics I posted are of Margo, one of Mother Russia's sexier exports. They were captured with a Canon 20D w/28-135mm zoom, ISO 100, f/5.6 @ 100th. I applied feathering techniques when capturing these. In the first image, I angled the main light source, a Mola beauty dish which has its own, inherent, feathering qualities and, in the second image, I used a monolight shot thru a Fresnel lens. As you can see, the light fall-off from the focused Fresnel is fairly dramatic.