A blower door is a special fan mounted with a frame and fabric panel into a standard (non-sliding) exterior doorway of your home. This does not require that the door be removed.
Controlled operation of the fan can create positive or negative pressure in the house. This slight pressure difference between the inside of the house and the outdoors allows easier detection of any leakage into or out of the house and the pressure difference exaggerates air leaks for easier detection and to help locate hidden air pathways through the structure. It also helps quantify the overall tightness of your home and may be used in conjunction with other diagnostic equipment.
If you have an open fireplace, please remove any loose ashes before the inspection so they are not disturbed by air from the chimney during operation of the blower door.
Contrary to what you may have heard, the blower door is not used to simulate weather conditions at your home, it is just a diagnostic tool for professionals trained in interpreting the results.