The process that happens the flue gasses are not able to exit the vent system so they back up into the home. The problem with this is that carbon monoxide occurs, causing silent and odorless poison. When this gas seeps into the home, it can kill.
Condition of flue gases failing to exit the venting system properly and instead flowing out of the relief opening of the draft hood and into the dwelling. Condition calls for immediate examination of appliance and venting system and corrective action.