The revs allow the fan on the front of the engine to turn faster, moving more air through the radiator. Driving down the road does the same thing - you are getting air flow through the radiator which is cooling it off. At idle you are not getting air through the radiator enough to cool it down. It should not be going up that close to the red line. Keep in mind too that the fans on the front of the radiator are two speed. Can you tell when they go to the higher speed?