When you "rev" a hot engine, a couple of things happen. If the fan is working properly, it will be clutched and will pull a ton of air through the radiator. At the same time, the increased RPM's will produce a higher circulation of coolant through the radiator. The sum total of all this is the coolant temp will drop at a greater rate than it would at idle. The increased air flow will also move the latent atmosphere under the hood as well, contributing to the overall cooling effect. Of course, none of this will happen if the thermostat is not working properly. Hope this helps..
1987 BMW 325
[This message has been edited by jeffsr (edited 04-05-2000).]