Read the slope where the ball curves, not the straight line
The instructor explains that looking straight down the ball-to-hole line is the wrong slope to read, because the ball does not roll straight to the hole; it swings out to the side on a curve. Reading straight line ball-to-hole means you are not perceiving the slope the ball actually rolls on, which is a big problem.
Channel: AimPoint Golf - Worlds Best Green-Reading System
Watch official source at 3:34