Drag Racing
Think of it as being at the races and checking the air with a weather station, it's at sea level. Your 409 has a certian amount of suction due to the piston moving down. The suction alows the atmospheric presure to push air into the cylinder, makes good power. Load up and head for Denver. Check the weather station, air is 5000'. Higher we go the less atmospheric pressure. Your 409 still has the same amount of suction as before at sea level, but the atmosphere doesn't have as much pressure to push the air into the cylinder. Less power, car slows down.
Shane