How does it work? One of two ways, but simple..
Method #1; The most accurate method is to input the cosine number into your ballistic software; i.e. Exbal Ballistic Targeting Software or Field Firing Solutions, that runs on either your home PC, Pocket PC or Palm Pilot. This is because the software takes into account the trigonomics of the fact that the bullet will still travel the full distance to target, and maintain a similar time of flight.
b. Due to discrepancies at shorter distance, we are advising all users of the ACI to only utilize method #1 and #2.
Method #2, is to multiply the cosine number to your sloped distance to target. This will deliver the flat line distance to target, or what we call the bottom leg of the triangle.
If you experiment with your ballistic software, you will find that there can be as much as eight (8) MOA difference between method #1 and method #2. Method #1 is the most accurate & preferred method.
Now, adjust the sight elevation knob (come-ups) on your scope, for the "corrected range".