How it works
Gravity works against your bullet over a flat line distance. On flat ground this is no problem, but when shooting at an angle, the distance in which gravity pulls on the bullet is actually less than the ranged distance.
Example: Your rangefinder estimates your target to be 400 yards. When you point your rifle at the target, the red line on the ACI reads 87.
0.87 x 400yds = 348yds
Your actual corrected distance in which gravity will be effecting the bullet is 348 yards rather than 400 yards due to the angle.