What is a blower door?

I am planning on getting an energy audit sometime soon to see where I can best save some money on my energy bill. I talked with an auditor who mentioned something about a blower door. How does this work? Is it safe to use on a home?
-Jim T., New York, NY

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A blower door is a fan that is used by energy auditors to locate and quantify air leakage in a home or building. It works by pressurizing or depressurizing the building in a controlled manner while measuring the flow through the fan. Air leakage, total hole size and other values can be derived from this flow. A number of other tests can be performed in conjunction with the blower door that can measure and test for duct leakage, air quality problems, and more. After weatherization work is complete, the blower door test can be performed again to demonstrate the improvement made and ensure adequate natural ventilation.
-Jeremiah A., Charlotte, NC