The bulge (heel-to-toe curvature) on the face of a wood causes balls hit off center to fly in the opposite direction of their spin. This is called gear effect.

To wit: A ball struck with the club’s toe will have left sidespin, resulting in a draw or hook. Because of golf club face bulge, the ball will start right of its intended target, so the sidespin sends it back on line. On heel shots, the ball will spin right; the gear effect causes it to start left.