当杆头偏离中心点击球, 即击球时候球的质心与杆头CG的连线没有与杆头的运行轨迹成为一直线,就会使球产生侧旋转.该效果被称为齿轮效果,在专利U.S. Pat. No. 4,471,961 to Masghati et al. 中进行了完整地解释

In essence, the off center impact of the ball causes the
head to rotate about its center of gravity and this rotation, or spin, is transferred into the golf ball, like two gears meshing, so that the ball spins in the opposite direction. Spinning balls curve as they fly through the air.为了补偿该有害的旋转,球杆头被制造成前突起的弧.

In order to compensate for this adverse spin, golf club heads have convex curved impact surfaces, known as bulge, that intentionally deflect the ball sideways by increasing amounts for progressively further off center hits so that the inevitable curve will bring the ball roughly back to the desired path in line with the direction of the club swing. This compensation is approximate at best, and many prior art solutions have been attempted to minimize the gear effect.