In car-speak, it's a loud, premature explosion in the intake manifold
... which is an 
explosion in a cylinder that happens when it's not supposed to, when the ... TOM: 
The backfire is recognized most often by the loud "ka-boom" it makes,... [DOC] [TLE] Engine exploded during jump start [Archive] - The Chrysler Minivan ... [PAR] I'm not an automotive engine expert, but for an explosion to occur you need fuel 
and a source of ignition.