It makes the air/fuel mixture richer or leaner from idle to 1/8 throttle, you can adjust it either way.
I had to bump my co setting up to 5 just to get my bike to start in cold temps after I added a pipe, programmer and opened up my airbox, I had it on 10 for a bit but the bikes snappier with it on 5.
With the stock setting ( 0 ) the bike would start then stall 5-6 times before idling on a cold day after I modded it, it was fine while stock tho.