Kicking Leg Tension
Leg tension is important in angular kicks, such as the round and hook kicks. In linear thrust kicks, such as the side thrust or back kicks, it is less important, since the mass of the body is directly behind the kick. In angular kicks, the mass of the body may only be applied to the kick through tension in the leg, since the mass is being applied at an angle to the kicking leg. Again, a strong musculature is required. If the hip joint flexes, power is lost. If the knee flexes, it may be injured.
Some kickers use a counter-motion when kicking. The counter-motion is to either lean the upper body away from the kick or to move the arms away from the direction of the kick. The reasoning is that this thrusts the hips forward to add power to the kick.
However, counter-motion makes no sense what so ever. It is at most a feeble attempt to maintain balance while thrusting the leg outward. In want sport does the athlete lean away from the direction a force is being applied? The mass of the body should be applied behind the kick or punch, not be held back, and certainly not pulled backward.
When you perform a kick, such as a side thrust kick, the arms should maintain their guard and the upper body should stay upright, or maybe kink forward into the kick. As a defense, you may choose to lean backward to avoid a kick, such as a round kick to the head, as you fire your side thrust kick.