The curve created by an off-center strike: when the ball meets the face away from the club's center of gravity, the head twists and the ball rotates the opposite way — like two meshing gears. A toe strike twists the face open yet tilts the spin axis toward a draw; a heel strike twists it closed and produces fade spin. Gear effect can overpower the swing itself, which is why a toe hit with a fade-shaped delivery can still draw. It's strongest in drivers and fairway woods, whose CG sits deep behind the face — manufacturers even curve the face (bulge) to compensate. When the sim shows a curve your path numbers can't explain, check the strike location first.