I agree with @Chef23 that watching a skier of the skill level of horton run the pass is going to be misleading.
Below is an email I send to most skiers after about 3 times out trying the slalom course. Frankly, for at least the first 3 times, I think it is best to offer essentially no coaching. I basically just tell them, "go out there, and enjoy getting your butt kicked by the course and see what you can do." Even with no coaching, I think of lot of light bulbs come on in their heads and they make really significant improvement from pass 1 to pass 18.
But now, after about 20 passes, most of them have had at least some success at like 26 or 28 mph and long line or 15 off or something like that, and they are probably going to hit a bit of a plateau and they know it, and I know it and so they start kinda looking at me like "dude, are you going to start helping me now or what?"
That's when I send them my standard canned email for course newcomers.
https://docs.google.com/document/d/1QuS3JTBfb1-EiCQ4mchHt3qehtBc8lZv29JjPHzw21g/edit?usp=sharing
It has contained in it a great article from @Than_Bogan in which he accuses them of "thinking slalom is all about the turns" and inevitably, they do. And that's the #1 thing they need to get over. They need to stop worrying about doing turns that look like the front of a magazine, and they need to start learning about stacked position.
That's literally it. I wouldn't coach a single other thing at this level of skiing other than stacked position, and let the turns take care of themselves. Just focus on stack stack stack. That's all we care about for this level of skier.
Once they start to realize that stacked wake crossings are 95% of what is required to run 30 mph -15 and the turns are only 5% of it, then they can start to have a chance at making this pass. Everything else is pretty much irrelevant at this phase of the game.