I was teached this way. Saenchai (Muay Thai) does that way.Flexing the knee and bringing the kick up more to the center before pivoting (your supporting foot is turned before you kick, which also telegraphs the kick) makes it more difficult to determine exactly what kick you're throwing. The longer it takes to figure out, the faster your kick is, effectively.
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I'm telling you how a roundhouse should be done by Taekwondo standards. There are differences between systems...
But, strangely to me, a lot of professional kickboxers and MMA fighters do it differently. So I have to admit that they know what they are doing and maybe there are different advantages and disadvantages of each one, regardless of the style. And I admit the way we advocate is not the easiest delivering power...