Stance is probably the most important thing in a spinning back kick according to most martial arts experts. You need to stand in a fighting position, with one leg a step in front of the other and your body should be tilted towards a side. Hold your arms in front of you.
The weight of your body should be evenly distributed on both the legs in order to have a perfect balance. Your eyes should be wide open, focussing on the exact target you want to hit.
Heel should be facing the target
Make the heel of your front foot face the target by turning your leg. When you practice it slowly, it is going to feel very strange. However, you will feel more natural once you start to do it quickly. You should regularly practice pointing the heel towards the target; otherwise, you are likely to miss.
While the heel is pointing towards the target, you need to shift your weight on the front leg. To get into an ideal position for the spin back kick, raise the knee of the other leg in the air. Turn your head to the other side swiftly. However, you must ensure that all your focus is on the target.
Remember that the weight shift, knee raise and the head spin should take place at the same time.
Deliver the kick
Drive your leg straight towards the target. You need to hit the target with the heel with immense force. The use of the gluteal muscles and the momentum make it an extremely powerful and effective kick.
Your follow through should be such that you move away from the target quickly and anticipate a reaction.