Technasonic Check-Go Pro Sweet Spot Electronic Golf Ball Liner Using physics spins golf balls to find sweet spot Marking pen included to mark the golf ball Batteries are included Locates both the ball's Balanced Equator and Sweet Spot in 30 seconds. High Speed 10,000 RPM Gyroscopic Action. Works with All Brands of Golf Balls. LED light indicates when the ball is balanced and ready to mark. Built-in Alignment Cup (to create your own partial or bold line).