The gear effect is real. When the head of a driver is hit off-center it not only slows down but it also rotates about its center of mass. Its this rotation that causes the face of the club to rub across the golf ball and give it the side spin that is said to be due to the gear effect. The side spin due to the gear effect is opposite in direction to the side spin caused by the face opening or closing during the collision. i.e. For a hit towards the toe the club would open up causing a slice but the gear effect would result in a hook. Which effect dominates depends on how far back the center of mass is from the clubface and the specific impact point. In the case of the woods the center of the club head is far from the clubface and the gear effect dominates. The clubface is curved to compensate. The closer the center of the club head is to the clubface the less the gear effect and therefore the less the curvature is needed. That is why irons are typically flat.