If you right click when equiped with flamethrower, a large amount of air will get out of flamethrower. This air has power to push enemies back, and people use it to throw people in cliffs and make them die. If timed correctly, this air can also push rockets, sticky bombs, pipe bombs etc back.
See this video it shows how to airblast(compression blast) properly
[YOUTUBE]iwLfGunep2Q[/YOUTUBE]
Backburner is an item/weapon you get by completing achievements(milestone one I guess is its name). This weapon gives you ability to cause more damage to enemy when attacking from back but takes lot of ammo when air blasting.
This airblast technique can also be used to play TF2 tennis (you were on that map, right?)
Wiki told me something I didn't know:
Khair manao don..