Gear Effect

According to TPI, if the golf ball impacts the club face directly in-line with the club's center of gravity (CG), there will be no resultant face rotation or twisting of the club due to contact collision.  If the ball hits more towards the toe than the club’s CG, then the club will react by rotating open (clockwise for a right-handed golfer) and vice versa for a heel contact. Gear effect is the counter rotation inflicted on the golf ball due to friction during the interaction of the rotating face on the golf ball.

On toe hits (for a right-handed golfer) the clubface opens (rotates clockwise), which causes the golf ball to rotate counter-clockwise in response. This resulting spin causes the ball to curve (to varying degrees) from right-to-left.

On heel strikes (for a right-handed golfer) the opposite is true. The clubface closes (rotates counter-clockwise), which causes the golf ball to rotate clockwise. The resulting spin causes the ball to curve (to varying degrees) left-to-right.

Because the center of gravity is typically located much closer to the club face in irons and wedges vs. drivers and metal woods, gear effect is much less pronounced in off-center iron and wedge shots. Drivers and metals are designed with bulge and roll, horizontal and vertical curvature of the club face surface, which accentuates gear effect and provides golfers with an added degree of shot correction on off-center strikes.