#combustionbending

First up, combustionbending is a very internal and almost spiritual type of bending. It's actually more similar to lightningbending than it is typical firebending. It is essentially lightningbending without the high degree of control it requires.

That isn't to say that combustionbending doesn't require control, it does. But, there's a difference between controlling your chi inside you and controlling it outside of you. Most basic firebending is focused on controlling the chi right as it exists the body to form it into a stream of fire or a ball, or controlling it once it is outside the body in the form of controlling fire burning a leaf, or influencing other fire sources around you. The only internal chi control required is just whatever is needed to guide it out of your body. It is only once you get more advanced that there is more emphasis placed onto controlling it within yourself, such as heating up your hands to heat up your tea (or otherwise just heating up your body). The technique of charging up firebending before releasing it is a mixture of this internal chi control as well as external chi control.

Lightningbending, on the other hand, places an extreme focus on internal chi control, as well as a decent amount of focus on external chi control. Internal chi control is paramount to separating the positive and negative from the chi, as well as guiding it out toward where you want it to come out of. External chi control comes into play in guiding it. While lightning can't be guided particularly well by most lightningbenders, beginners still have to guide their lightning into a stream. Slightly more advanced lightningbenders can form it into a ball before releasing it.

So what is combustionbending then? It is somewhat of a mix of firebending and lightningbending. Unlike lightningbending, it involves no separation of chi, however it is extremely internally focused. Take Ozai's "seed of fire" used in Sozin's Comet to burn down the Earth Kingdom. He is gathering his chi externally in one spot before releasing it. Combustionbending is the same thing, but internally, resulting in the area closest to where the chi is being pooled becoming superheated before it is released. It involves little to no external control-- primarily because it is extremely hard to control. Think of it as trying to divert the path of an airplane. Skilled combustionbenders can curve the path of their bending, but thats about it. Unlike firebenders or even master lightningbenders, combustionbenders can never create a wall of combustionbending.

It is, for the most part, an innate skill, and an extremely rare one at that. It must be learned and developed, but the average firebender cannot learn combustionbending. Combustionbending requires extremely stable chi, as any degree of instability within the chi (instability being defined as the spontaneous separation of positive and negative chi) will instead come out as lightning, which requires external control or it will explode in your face (a la Zuko). This means that lightningbending and combustionbending are inherently at odds. A lightningbender could never combustionbend. A combustionbender could never lightningbend.

But can combustionbenders firebend?

Well, yes and no. Combustionbenders tend to start off as firebenders (often prodigious firebenders, given the extreme stability of their chi), however the process of becoming a combustionbender involves blocking off all chi routes (through meditation or other means, often against the wishes of the bender) until the chi is forced to be guided toward the head. This is because the hands, feet, and mouth are all natural places for the chi to tend toward, and it will naturally exit all these places as fire. The forehead is the only place that it will not exit and can be pooled to create the desired effect. If a firebender were to attempt to pool their chi there, they would find it naturally escaping through their nose or mouth.

The tattoo is not necessary, but it signifies the completion of the process of blocking off all other chi routes. It is comparable to an airbender receiving their tattoos to signify their mastery.

In gaining combustionbending, they sacrifice their ability to firebend.

Combustionbending training involves a large amount of meditation, primarily before the chi routes are blocked, but also after to ensure none of the routes reopen. If they do reopen without the combustionbender realizing, it will cause the combustionbending to fail and can also cause damage wherever they reopened, as the paths have often been unused for years.