When your about to have sex with a woman doggy style and the woman has a fat ass. To reach the pussy you must lift up her butt cheeks and fly under the radar to have sex. This can also happen when a woman have a f.u.p.a and you have to lift the f.u.p.a up to reach her pussy.
Greg: Dude your girlfriend has a fat ass! What is that like when you bang her from behind?
Jack: Well I have to Fly Under the Radar to reach her pussy.
Greg: Oh My God.