It's about a change in speed. If the cue ball and the first object ball are close, use 2 rail speed. If the distance between the cue ball and the object ball is normal, use 3-rail speed. If the cue ball and 1st ball are far away, use 4-rail speed. If you want to increase the speed by 1 rail, just set the ball value to minus 1. This is what it says!