When near idle speed lo range crawling with no airflow thru the radiator the cooling fan will run frequently, as compared to not at all when highway driving. I highly doubt hill decent will overheat the brakes using it often since you are only going a few MPH when in manual mode lo range using it. It keeps your auto trans from free wheeling and rolling down a hill to fast. As soon as you touch the gas it defeats it then you let off and it works again (so don't touch the gas or brakes and let it do its thing using the shifter to speed up or slow down) other wise it could lurch/speed up when not wanted. The fan kicks on @ about 224-226 deg on my 2.0T.