You don't want to keep it dead leg with no tension. You lead with the hips but your extend the leg with tension at the point of impact.
For the sake of explanation let's say that you are doing an outside round with your right leg (back leg)from a right handed stance, to the thigh (a cut kick).
You begin by rotating your left foot and twisting your hips. Right leg comes up and you have bring your right knee to hip level, naturally your right knee should turn into your opponent. As you rotate you extend your leg, for the purpose of this kick you begin to bend your left knee, and angle your right knee downwards. The idea is at the point of impact your knee is slightly bent, the tension is coming from the fact that you are doing a dynamic extension(kick), but that it is driven, rooted, and lead with your hips.
A cut kick comes up as your rotate then down, you are essentially sitting into it with allowing your body weight to assist. Similarly you "sit" in on most boxing punches, meaning a slight drop in your center of gravity usually by bending the knees that helps put more weight behind each strike.
High kicks are done differently, only in the fact that you don't sit into it and you continue the natural rise with your leg and extension.
Without the kick extension you won't finish 360 degrees, the kick gives the final moment for the turn. If it is just hip driven with a dead leg, then you only end up about 90 degrees.
The common problem really becomes people mistiming the extension of the kick, or trying to kick too hard and ending up with the foot/shin ahead of their hips at the point of impact.
Forceful extension really occurs maybe 6 inches or so before point of impact, essentially when you reach 45 degrees of target your begin your extension. So that at 90 degrees you should be kicking through your target/point of impact.
Hope that isn't too technical.
Point being your leg is not dead, it starts dead to bring it into motion but your hips lead and leg connects with an extension. Sort of like a golf swing.
It is a lot like a boxing punch in that it is hip driven and then the extension of the striking point. The goal is to get as much body weight and torque behind the strike as possible.
Hope that helps.
first round house is not a rounded kick.. the basics are in a foot kick.. instead of you keeping your back foot strait you turn it then lead with the hips.. you should never do a 360 for a round house.. so your basic round house should have a strait forward motion never coming from the side knee up and back foot turns to a 90 degrees and force your hips into the target with the shin and ankle making contact with the target.. its easier to learn low at first then high so you learn to put power into it..
Ok basic round kick using the hips spins you around 360 degrees if you miss.
You know how you can rotate the hips by rotating the foot and legs and then the hips like a boxing punch?
Also you know how you can rotate the hips alone and the leg is dead, well it is somewhat effected since everything is connected it would rotate but via the hips not via the leg muscles, but its mostly dead and the hips are rotating in isolation as much as is possible. (like a karate punch)
(The boxing punch is harder than the karate punch)
Butl for the thai round kick should you let the leg be as dead as possible so its looser and can create more speed? Or should you have some tension in the leg as you rotate the hips and legs so you get more muscular force into the kick?
