Technically, it is suppose to boost contrast in low contrast areas (i.e. white wedding gowns with subtle transitions and the like) I mainly use it when there is a lot of sunlight that washes out certain areas of an image. Dunno if it really works since I have not done a head to head comparison. It does slow down burst mode shooting so I have it as a custom function setting on my 40D when and if i need. Otherwise it is set to off.