A airblast is when you right click on mouse when using a flamethrower. On normal flamethrowers, a airblast would cost you 20 ammo (or 50 for the back burner). But there is flamethrowers that you can not use the airblast. That flamethrower is the phisoliter (how ever you spell it).
A airblast can be used to do many things, which I'm going to show you in the next chapter.

A airblast is used as a weapon, or a not so weapon (for your team).

So first of all, the airblast can be used to deflect projectiles. A projectile is like a object that is thrown or shot out, and not at like the speed of lightning. That means that a sniper bullet or a minigun bullet is not a projectile. What is a projectile is like a rocket or a arrow or like jarakade, gerenades, sticky bombs, etc. To deflect they, or shoot them back you have to airblast them before they explode. It is possible to deflect them back at the enemy and kill them. Also you can rocket jump by deflecting a rocket so that it goes at your feet and do the normal rocket jump (but it is very complated)

The next type of airblast is when you airblast a enemy in range or the airblast, and deal knock back to them, which is useful is you are running away. You can also use it offendly by knocking them into the air and then shooting them. They probably will be confused, and so you will have a head start. You can also shoot them with the reverse shot (I think) and since they are/were aireborne, it should do more damage (or something). You can also knock them our of the world, or make they die from high places, for example the map pr_hightower, etc.

And finally, you can airblasts on you teammatees that are on fire so that you can put the fire out. It is extremely useful on gamemodes which your teamsmates are toghter (payload, capture point,etc.) because if a pyro sprays them with fire, you could should airblast them and they should become nolonger on fire.

I hope this guide helped you, and if it did, please like it or leave a comment below