The clubface's journey from opening in the backswing to squaring back up through impact, driven mostly by forearm and wrist rotation. It matters because face angle at impact is the dominant influence on where the ball starts — TrackMan measures it relative to the target line, and the ball launches very close to wherever the face points, so a face just a few degrees open or closed is the difference between fairway and trees. Players manage the trade-off differently: an actively rotating face can square from wide open but demands exquisite timing, while holding the face more stable with body rotation gives up some speed of closure for repeatability. Slices and hooks are, at bottom, face-rotation stories.