Opening the book…
Forward shaft lean reduces the loft the ball actually sees, which is what produces the compressed, penetrating flight and the ball-then-turf order. It also makes distance repeatable: the same club delivered with the same lean gives you the same number, while variable lean gives you a spread. Lower launch with more spin holds greens better than the weak high flight you get from a vertical or backward shaft.
Film your swing from face-on at 1000 frames per second if you can, or just from the side on a phone, and check the grip end at impact — it should be nearer the target than the clubhead. Compare flights: your 7-iron should fly lower and land harder than it does when you scoop it. Trap-drill it by hitting balls off a slightly downhill lie, which forces lean naturally.
You need enough lean to de-loft, not so much that you kill flight and lose stopping power on a firm green. Wedge shots that must land soft, and shots played out of thick rough where the club must slide, call for less lean.