Back Side Distortion Factor. The phenomenon occurs when you see a female from the back and she is stunning. This could be because of the way her hair is done; or, more likely, how nice here booty is. Then she turns around and she is not only a tad on the chuncky side, but has a face that would make the most hearty of men run in fear.
"Wow, check out that girl at your 12oclock. She is FINE!! I hope it's not BSDF."