As a playing aid the Line-M-Up conforms to USGA rules and is legal in competition to help you align your putts and tee shots to improve your score.
As a training aid the Line-M-Up gives you a visual guide when putting. When a good stroke is made, the line on the ball appears straight as it rolls. When poorly struck, the line will appear to wobble. When a Pure Stroke (no wobble) with the proper speed misses the hole, you can identify a missed read to improve your green reading skills.
Green Keepers Line M Up Golf Ball Alignment Tool w/ Bonus