The two rods hang perpendicular to each other due to gravity, regardless of the slope of the green (patented feature). When you look over the trainer and only see the black rod bisecting the ball you know your eyes are directly over your line of putt. If you see any of the gold rod, then you are standing either too close or too far away from the trainer. Single rod or single string trainers can give you the impression that you are over the ball but because there are only two reference points (the single rod and the ball), you can actually be several degrees away from your aim line. For every degree that you are away from over the ball this can cause you to be approximately half an inch off your aim line (may result in lip out or ‘burning’ the edges).