There is a question on the Propane Pump Jockey test which was being reviewed in a room where I was internetting one day.
What happens if you get the ideal mix of air and propane and there is a spark?
Before anyone could say anything I piped up "You land two counties over"
The instructor (once the laughter died down) said "Exactly". (hey I got it right)
To make it burn you need the proper ratio of propane to air.
With the pressure in the lines being far far lower than normal (normally it is 11" of water or roughly 1/2 PSI) you do not get the proper air/gas mix and it does not burn.
But get the right mix and...."She flys through the air with the greatest of screams. Of course she has not a trapeze. Her movements are not graceful and no one is pleased.. For she has been blown away.