Rule 11 of 25 · Chapter II — The Strike
Deliver the shaft leaning forward at impact so the ball comes off lower than the loft stamped on the club.
Why this rule exists
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.
In practice
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.
When it doesn't apply
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.