What is a Hydrocarbon Flash Fire?

Flash Fire defined by CGSB 155.20-2000
" A Rapidly Moving Flame Front which can be a Combustion Explosion. Flash Fire may Occur in an Environment where Fuel and Air become Mixed in Adequate Concentrations to Combust. Flash Fire has a Heat Flux of Approximately 84k W/m2 for Relatively Short Periods of Time, Typically Less than 3 Seconds."