I found the article a bit confusing, due to the initial discussion about lower back curve (inward) but then a series of photos that seem to focus on mid back curve at the top of the backswing from a caddy view rotated.
The different options for leg work, such as keeping a flexed right knee on the backswing or straightening the right leg would seem to have an effect upon the lower back to some degree, or even quite significantly.
Another thought I had when reading the article was whether this is geared toward a power move only, or for precision ball striking. Player vs. Long Drive Champ. Very different types of swings generally speaking.
I tend to be a bit skeptical about an article that is focusing on everything about turn, coil, and loading up the muscles like a spring to unleash an assault onto the golf ball with no discussion about what is going to happen after impact. If misinterpreted, this can often lead to massive over acceleration issues, loss of shaft flex, and inconsistent ball striking.
I did see the mention about the unnamed tour pro who was faulted for driving his legs toward the target, causing a hip stall…
however, I think this has more to do with improper ground pressures which inhibit active torso rotation post impact. The player in question clearly loses spine tilt, and fails to properly load into the right foot on the downswing, blocking off the arms from getting to a flat entry along the 4:30 line.
A lateral movement of the knees through impact can be very good biomechanically, if done like Tony Lema. Tony had fantastic torso rotation ontop of firm and braced knees through impact.
Lema’s left knee is like a rock… right through the frames, establishing low point quickly with the torso grinding actively on top, combined with fantastic hand action… ultimately holding shaft flex… top stuff… Lema won the British Open in 1964, coincidentally at St Andrews.
I disagree that a lateral leg drive is bad methodology, even though I don’t employ that significantly in my own swing. I probably should.
Fully rotated shoulders on the backswing, turning around proper spine tilt, should put the weight over the right ankle, and see the player in a good transitional pass through at the top of the backswing. This should encourage what the author is getting at.
From there (the top), cupping the mid or lower back could be beneficial, but only if the torso is working properly post impact…
otherwise, we likely have another long hitter reaching for another golf ball… and not a pure striker like Lema holding shaft flex.