It activates during cold starts. It closes to put a load on the engine and help it warm up faster. The problem is that until the truck reaches full operating temps it will close sometimes while the truck is moving. It kills your milage when it does this, and the truck is a little low on power when it is closed.
I have it closed all the time in one of my high idle settings, and it is deactivated in all of my other settings on my chip. Except one more. It can also be used as an exhaust brake. (decel tune) Works great