Student or Learner
Narrowing his eyes against the heat, Eragon watched with satisfaction as the egg burned. [...] When the screaming ceased, he extinguished the flame, and it went out from the bottom up.
(C. Paolini; Inheritance)
Does the part in bold mean 'completely'?
Gas is lighter than air.
As gas burns, it burns from the bottom up.
If you cut off the source of fuel (gas) the remainder will appear to burn from the bottom up.