What I have found over the past year of intense commitment to S&T is there are a several fundamentals that, for me, highly impact my swing, and whether or not I play very well, or average. Most of the issues revolve around improper weight movement during the swing. If I were to setup a machine to hit a ball at the same bottom point every time, the machine would be placed on a solid foundation, not on sand, and the internal mechanisms would be balanced such that the movements would not create an imbalance or vibration during movement (like an out of balance dryer). The "feet" of the machine would be wide enough to maintain balance and harness the internal power generating movements.
Trail foot position: Coming from the conventional swing, I am haunted by weight shifts to my right side on the backswing. Added to that, my historical trail foot default position is nearly 90* to the target line vs flared. A straight trail foot position makes it extremely easy to get my weight over my trail foot, and worse, on the outside of my trail foot. When my trail foot is nearly straight, it's much easier to transfer weight to the trail foot, which moves the bottom of my swing to the right, and can result in shanks, thin, fat shots. When I flare my trail foot it's far easier to keep the weight on the inside of the trail foot. A flared right foot is "weird" for us traditional swing folks, but now it's a must. Width of the stance also plays a key role in remaining stacked over the ball. A narrow stance makes it easier to get onto, or over the trail foot.
Shoulder turn on the takeaway: I have found my "lazy" backswing initial move is to start with a 2 inch lateral slide of my shoulders on the takeaway, then turn. The problem with that is, 1) my shoulders flatten, 2) my weight shifts to my trail side, 3) my head moves to the right (thus also my weight), and 4) my shoulders don't turn properly, or I end-up with a 45* turn, not 90*. It happens very quickly and can go completely unnoticed. I can't count the times I chased other "fixes" that weren't broke because this bad move creeps in and can be very subtle. The fix is to get the left shoulder moving downward immediately upon takeaway. This also gets the club up faster on the backswing vs. deep and lower. When I do this correctly, I feel tilted to the left at the top, but I'm not. Yeah, feeling tilted at the top feels weird, but it's a must.
Takeaway too outside: As noted above, when the left shoulder moves laterally on the takeaway, vs. downward, the club is pushed outside on the takeaway, and too deep (more like an oval-ish swing vs. circular). For example, if someone was standing directly behind me, and they wanted to touch the club head halfway back, if I laterally move my shoulders then turn, the person behind me would see the club head being lower and to their right. When my left shoulder rotates downward on the takeaway (not moving laterally), the person behind me would see the club head directly in front of them, the head would be higher, and similar to if I was trying to pass a baton in a relay race to someone directly behind me.
Downswing Start: It's also very easy to initiate the downswing by moving my shoulders leftward first, or my shoulders are doing the work that my lower body should be doing. This move results in excessive weight shift ahead of the ball, and "drags" the club at the ball. The result is thin shots, fades or blocks. Again, this move results in too much weight moving too early onto my left side. I'm simply too far ahead of the ball.
In summary, I'm finding that monitoring of my weight distribution is the most critical component to my swing. Any moves that shift the weight outside (left or right, forward / back) vs. being stacked over the ball will negatively impact my swing. The way I know if I'm getting out of balance is by impact feel and ball flight. Fat, thin, or shanked shots mean my weight is on my right side. Thin and "smothered" shots mean my weight has shifted too far on my left side, such as starting the downswing with a leftward shoulder drift vs. rotational downswing.
In @TomSaguto 's online class re: hitting long irons and hybrids (https://saguto.golf/courses/1161820/lectures/24965593) he points out that in order to decrease the increase in out to in swing path created by moving the ball at address a bit closer to the lead foot (which can cause additional left to right side spin), one should put a bit more weight on the lead foot at address by a small hip slide toward the target.
My question is, why do this vs drawing the trail foot back some from the target/foot line? This has always been the correction recommended in traditional golf instruction, along with using a stronger grip for any desired reduction in left to right side spin.
My guess is that S&T wants to do as little as possible to change the basic set up and by extension, the swing...but I just had to ask!