It's like adjusting the vents in your house. The closer vent to the furnace gets the most heat, so you reduce the vent to control the amount of heat. Further away from the furnace, you open the vents to allow more heat. Adjust the tuning plates to evenly allow heat to spread out throughout the food chamber. The futher away from the firebox, the wider the gap between plates.
Performer, UDS, 2011 WSM, Frank-in-Weber, 120 Gallon Offset, IQ-110, ET-732