Most people that have been modding computers for a while, know that rheostat's are variable resistors with a wattage rating. There are a lot of schematics on the internet that will show you how to use a rheostat to control the current going to a fan. There is a downside to resisting the current directly by the rheostat and that is HEAT!
When a rheostat resists current (Amperage), it will convert the current into heat causing the rheostat to get very hot, often too hot to touch directly. Although this isn't necessarily bad, there are better methods of variable resistance fan control, such as using a voltage regulator with heat sinks and potentiometers like Sunbeam has with this Rheobus.