A Flying Back Kick (also called a 360 Back Kick) is a variation of a Jump Back Kick. When naming kicks, the distinction between a flying kick and a jumping kick is that the term "flying" typically denotes that the body has significant forward momentum while jumping, as if from a running start. The same is true of the Flying Back Kick. But how can you get a running start, for a kick that's done backwards? Answer: You begin running forward at the outset of the kick, then jump in the air. While in the air, you rotate 180 degrees so that your back is to the target, and then kick.
As with all Back Kicks this is a linear kick, meaning the kicking leg does not arc its way to the target. Rather, the leg is thrust linearly at the target while the body is still in midair.