and why you should play persimmon When impact occurs toward the toe, the club head rotates to the right around its center of mass - imparting draw-spin to the ball. This is known as the "gear effect." Our SMART Spin Driver is designed to maximize the gear effect. In chapter 36, titled "Golf Wood Club Design: Horizontal Face Bulge", of Ralph Maltby's Golf Club Design, Fitting, Alteration & Repair, Mr. Maltby describes the performance characteristic of bulge as the built-in correction or compensation factor which under normal circumstances at impact starts the ball farther to the right on toe shots and farther to the left on heel shots in order to compensate for ball sidespin caused by such off center hits. Mr. Maltby uses the term "Gear Effect" and describes it like this: If the clubface and the ball both had teeth, as would be the case with two gears meshed together, then rotation of one gear in a certain direction would cause the other gear to rotate in the opposite direction. Very simply, the bulge of a wood club allows toe and heel shots to move toward the center line. The Gear Effect then gives golfers increased forgiveness off the tee. Wood clubs have greater Gear Effect because of their roll and bulge coupled with their deeper center of gravity. This gives you an accuracy advantage against any metal "wood" on the market. With a persimmon wood, spin is your friend on mis-hits; it not only brings the ball back towards the fairway, but it cuts back on the distance of errant shots. Metal "woods" may give you more distance on mis-hits, but it is off-line distance that is no good to you and detrimental to your accuracy. SMART golfers play with spin and the Gear Effect. SMART golfers use persimmon wood.