As Robert said, the main effect is from the top vent. But the bottom vent controls how much charcoal is burning (i.e., air inflow). If you want to ramp up quickly, then open it a lot. Once you get to within 50F of your target temperature, you can dial the bottom vent back to conserve charcoal. The KK is very efficient, so once you're at target temperatures, it doesn't take a lot of air flow to maintain it.