What is gear effect
"Gear effect" is the term used to describe the action of the club head, during impact with the ball, that causes a shot hit off the toe to curve in a draw or hooking motion, and a shot hit off the heel to curve in a fade or slicing motion. This is far more the case with woods than irons and that is why you will see the club face of a modern day driver have a convex bulge on it
The gear effect is also the reason that all wood heads are designed with a horizontal curvature across the face (called "bulge").
For a right handed golfer, if the contact is made from the toe side of a driver, the club face and the golf ball will work as two gears and give reverse spin to one another. This shot will actually start to the right side of the target and then have a right to left draw spin on the ball that will make it curve back to the centre of the fairway. Thus the bulge on the club face causes the ball to start off more to the right, after which the hook spin generated by the gear effect takes over to bring the shot back toward the centre of the fairway.
For shots off the heel, the bulge across the face of the wood causes the ball to take off to the left (for a right-handed golfer), after which the fade spin generated by the gear effect takes over to bring the shot back toward the centre of the fairway.
These actions of sidespin and shot curvature happen because the club head rotates around its vertical centre-of-gravity axis whenever the ball is hit off the toe or heel.