This is a back kick and beginners may mistake it with the roundhouse kick. But this is different. The latter is a straight, sweeping, circular movement. The former involves turning back midpoint almost. However, though not as dramatic as a roundhouse kick it is powerful too, because there is less effort invested in maintaining balance which may be distracting if you attempt a roundhouse kick at the end of a match! But in the back kick the surprise element is ensured, as in the roundhouse kick, yet  your base remains strong. Your target is met mid-body and your balance can be restored pretty fast. And continue facing him, after the kick, which is what the whole thing is actually about -- facing the target headone.

As you can see I am using the edge of the foot to deliver the kick. The key element is to maintain the balance of the other leg, which equally contributes to the kick by providing a great balancing support and you land back pretty far off the target to avoid him coming at you, if he is a strong fighter. In all kicks the torso moves back but this does not involve bending the supporting ( a mistake beginners make)  leg which will unnecessarily shift the pressure to the back, contracting it instead of releasing it.  There are two reasons why your torso shifts back to deliver a kick: one is that you are moving away from contact point with the target who, if strong can punch or kick you back. Plus, the thrust of the kick needs this torso shift. Basically and very simply for this movement to be complete, precise and perfect, you need to have strong legs. That way the supporting leg can take any swing of your body. The movement is supple not stiff. A stiff backward drop -- again a beginner's mistake - will jerk the neck, contracting it. Avoidable!

Watch the landing each -- legs wide and away from each other, to maintain your base. A gentle tapping, bouncy leg work, to keep yourself ready and loose for the kick and any subsequent response from the target. The leg work is very important, btw. to maintain your balance continuously. Leg work training is a work of art and actually defines your fighting skills because it will define your power, your balance and your stamina. You need to invest in it, for a perfect back kick.