Adjustments to the needle height changes the power in the mid range RPMs.
Like any other carb mod, you have to be careful with it, going too far makes the bike run poorly.
The thread "Carb Tuning" explains the relationship between pilot jet, needle, and main jet mods.
The nice part about carb tuning is there is no permanent change to the bike, and you can go back to stock as quickly as you modded. (but you won't )