A spin kick follows the rules of uniform circular motion. When doing a spin kick the foot moves tangential to the circle while rotating about the center of mass. To achieve a spin kick you must apply a force greater then the net force pushing downwards which are the force of gravity and the mass of the legs. If you don't apply a force greater then the net force while trying to making the turn in the spin you will fall over before completing a spin. To overcome the force and maintain a spin kick is by applying torque (Torque = lever arm X force) to your legs, which is a twisting force that rotates an object around a axis point.  What makes the round house kick so deadly is the power (Power=Torque X angular velocity). To maximize the angular velocity you must spin at a certain angle and know how to use your arms to increase and decrease your moment on inertia by moving your arms in a certain pattern while spinning. The torque of the kick is limited to the size of the fighter's leg. So, to to achieve the most power by the spin kick would be expanding your legs to the max extent when kicking and timing your arm position when turning.