The spinning backfist is something of a “high risk, high reward” move, in that it is very powerful if connected, but if it is missed, it will likely result in you being clowned by your opponent.
While, in martial arts situations, I prefer to keep things simple, the spinning backfist is a move worth knowing, even if its only to know how to defend against it. And, a quick video search will find examples of people being knocked out by this move, so this is more than just a technique for the movies.
It seems daunting, but it is really not so complicated. The bulk of the work is done in the legs: swing the back leg and the torso in a circular motion towards the opponent, in the same direction as the leg being swung (ie: the right leg and torso spin clockwise/to the RIGHT if you are throwing a right spinning backfist, and the left leg and torso swing counterclockwise/to the LEFT if you are throwing a left spinning backfist)
As you spin, your head looks over the shoulder of the arm that will ultimately punch. Once you see the target, throw the arm and land with the back of the fist.
For those who prefer a visual, observe the video on this very same subject.