Sanjib - it's not hard at all. Select camera settings to ensure that the faces are properly exposed. In a "classic" backlit situation, you will find that the background will be quite light and possibly even blown out. None of Bill's images are purely back lit as the light is often coming from a side as well.
Diffuse lighting as in the 4th and 5th image will also ensure that there is light on the face.
If you are looking at pure backlighting with strong light coming through a window or from a flash modifier, you will get a pure black on white silhouette with no details on the subject.
You can't tell, but the model was looking right at me when I took this shot. You can see her outline, some hair and the delicate materials on the sleeves of the top she was wearing.
