The problem with this technique is that it is only temporary. There is still stress in the metal and what will result is a condition called "oil-canning". This is where the alignment of the molecules is not correct which results in weakness (fatigue) in the metal. The next extremely hot or cold day the dent will reappear.
it popped the dent out! physics! First, he heated it all the way thru to the inside surface with the heat gun, then when he chilled the outer layer with the compressed air, what resulted was an expanded/heated inside layer pushing and the contracted/chilled outside layer pulling. Any high school science teacher should be able to figure this one out.