I don't know where to put this. This needs lots of documentation and analysis, and this is just a start, only notes on something important that Sylvie discovered (and I guess I discovered in watching and talking with her) about the "Thai Kick" that simply evaded our perception for years. 5 years in Thailand Sylvie's been working on her kick, she didn't see it. Years before that, didn't see it. But then in two Patreon Muay Thai Library sessions, neither of which have been published yet (one with Karuhat, one with Sagat) it rang like a big bell. And it became incredibly obvious. Sylvie talked about it a little bit on the Patreon Supporters Facebook Page, and a commentor called the kick the "Golden Kick", so I'm going with that, as it's reflective of a lot of the technique of the Golden Age of Muay Thai. And, as I've begun training in Muay Thai I've been working with it myself, as a knowledgeable beginner, I'm interested in this kick as some of principles below have definitely changed my own training and though process. In any case, here are some starting notes. Hopefully this will be built out by Sylvie herself, and maybe some more film study:
The above is a big exaggeration of how the "Thai Round Kick" has been, I believe, distorted in its translation to the west. Part of the problem perhaps is that it is often referred to as the "round" kick which certainly triggers us to think of roundness in the kick. The name inspires us to think about how it goes 'round the opponent, from the outside, at least in certain teaching contexts. Also, there is something of an optical illusion in the Thai kick itself. Because it has that swerve after forward momentum I believe it kind of looks like it is going "around" or outside the opponent, when seen at full speed, when instead it is often much more running up the frame of the opponent, and then veering through. First of all though it should be said: there is no absolute "right" kick. There are innumerable Thai techniques for kicking, and there are kicks for different purposes and situations. And there are many other ways of kicking as well, developed throughout the world. All that being said, The Golden Kick, as it's been christened, is very distinct in one way. It tends to move upward, and relatively straight-forward, and, to some degree then makes a hard turn when it accelerates through the opponent, at the end. Its upward, straight-ahead approach to the opponent makes it extremely fast, and also hard to see. If you think about it, anything that breaks your "frame" visually the eye, just by how it has evolved, will pick up as anomalous movement. The wider, early acceleration kick that westerners tend to use is quickly seen in its arc. By keeping the kick closer to the frame it is detected much later, often too late.
Below is a rough cut of various Thais kicking across decades. Keep a close eye on the early trajectory of the kick. In all these variations it comes up and forward, even though these kicks are thrown in differing circumstances and tactics by very different fighters. The misses and pulled kicks are sometimes even more telling so they are included. Once you start looking for this you see it in lots and lots of kicks of the Thais, especially in the Golden Age. What's even more incredible is that these aren't "cherry picked" kicks, trying to illustrate a principle. I just looked through fights of well-known fighters and grabbed any round kick that had a clean, visible or instructive angle. The compilation creates the principles.
I know Sylvie has experimented with this already in the last few weeks and has suddenly been able to kick one of her trainers who she never could kick before. He used to taunt her and say he'd give her money for any kick she would land. Now they land at will.
In any case, this isn't an expert's post. This is really an enthusiast who is simply suggesting a change in how you think about your kick. In the graphic everything is exaggerated. More important perhaps is how you think about your kick. Don't think about it going "out" as if you are starting an arc. I believe part of the problem with teaching the Thai kick came from very helpful analogies to swinging a baseball bat or an axe. I know we really fell victim to this.
above, not like this
This image is helpful when thinking about how clearing your hips generates power. But the way it is not helpful is that it creates an arc that isn't true to how many of the Golden Age throw their kicks. You don't want to "swing" your kick as if you are hitting a baseball or chopping a tree. The kick comes directly upward, straight to the target (in some fashion, there are variations), getting as quickly to the hitting zone as possible, and then swerves, accelerating to do damage. It's one reason why Thais are so hard to read and seem like they can land kicks through the guard at will. There's a lot more to dig into here. The forward trajectory allows Thais lots of nuances in how and when they decide to swerve the kick over and through the opponent, ways of generating power with hip angle, choices in how to combine or vary the rising vs the cutting action, games they can play with timing and perception (the coming Karuhat Muay Thai Library session is incredible in this). This is just the beginning.
If you want a good example of the opposite of this kick, a much more western style kick, look at the kicks of Ramon Dekkers.