Occurs when an incandescent light bulb of greater than 25 watts is heated by the fire, causing the gas inside the bulb to expand and push out the softened glass on the side closest to the heat source. The portion of the bulb that bulges out will point in the direction of the heating. Incandescent bulbs of less than 25 watts do not have gas inside; they have a vacuum. Therefore, if heated, they push inward on the side closest to the heat source. If they survive the fire and have not been touched, "pulled" bulbs can assist the investigator in determining the direction of fire spread.
For more information on this term, see the interFIRE VR Resource File article: Examples of Thermal Effects on Materials