We are almost at the stage where the international community agrees on the definitions of Flashover and Backdraft. Both of these fire development "events", occur in the room where the fire is developing (or just outside of it).
Sadly, this is where most fundamental firefighter training programs end.
Much to my frustration, I continue to see our colleagues being caught in extreme "changes in conditions", while engaged in activities in rooms that NOT fire involved. They are working in a room that is remote from the main fire compartment. Smoke has accumulated in this area, or in concealed spaces. As it "rolls across" the ceiling, it is diluted (partially pre-mixed) with the cooler air in the "under pressure zone".
When firefighters enter these "adjacent rooms", they encounter a smoke layer that has cooled and thinned. This gives a false sense of security. The reality is that when "fuel is pre-mixed with an oxidizer (air)", it will burn more efficiently. If the ratio of fuel (unburnt) to air, approaches the "ideal' or "stoichiometric" ratio, the result can be extreme (very high temperatures and explosive pressure increases).