I thought I understood this setting. I am using an Sb-900 primarily for fill flash. Previously Nikon cameras would not allow separate settings for flash and exposure, they were tied. E 4 allows that connection to be cut. However, I am not sure even after trying if I can detect a difference with fill. It seemed logical to divorce the two settings, and use the flash compensation to keep the fill at the lowest possible level. I suspect TTL is compensating for the compensation. Which means this setting may only be useful for straight TTL and manual flash. Any one else playing with this setting?