The side kick is a difficult kick. I feel GM Kang was saying "if your side kick is good, your overall proficiency is probably good."
Like a Little Leaguer's fastball. A lot of good young players can develop very capable curveball pitches, knuckleball pitches and so on—but the recruiters are looking for the youngsters with blazing fastballs, because if you have that, pitching is pretty much a natural gift you were born with. I remember long ago reading an interview with Whitey Ford in which he talked about that... WF said that the fastball is the one pitch you have to have been born with. And he should know—he
didn't have a doppler-shift fastball and had to compensate using other kinds of pitch...
The sidekick is a technically very difficult kick: your upper body has to be making very fine continuous adjustments in position off the vertical, smoothly, to maintain balance all along the length of your leg extension, with a very sharp hard jolt at the end, and that leg extension requires very strong hip flexors and abdominals, as was pointed out by a couple of posters earlier. It's a brutal test of strength, balance and accuracy all at once. That's the reason why I work on it so much, I suppose. But I see the benefits of that strength, and those balance and accuracy skills, being cashed out in the availability of low, hard, damaging short-range kicks in SD situations.
In a way, the sidekick is the ultimate exercise: it's kind of to TKD what the bench press is to weightlifting. (Or the squat, for you quad fanatics!

) Terrific strength developer, but what you're really after, if you're like most people, is
functional strength. I think of the side kick more along those lines—as a rigorous drill that stresses all the major requirements, and as a great demo/showoff movement; but the real payoff is the application of that strength, balance and accuracy to survival in a down-and-dirty unsought combat situation....