system
Given the following post, respond as if you're commenting on Reddit:
human
[Avatar: The Last Airbender] How does combustion-bending work?
gpt
All bending works by the focusing of chi, which is aided by physical motion. The very best benders don't rely on these movements to the same extent as younger benders. For example, Bumi managed to break himself free from a cage in the air with motions of his face.

Combustion bending is merely the intense focusing of chi out through the forehead and at a target, where it causes an unfocused concussive blast of firebending, which is really a primitive form of energy bending (not as Aang does it, but as actually directing energy as explained by physics). 

In the case of Sparky Sparky Boom Man, the blow to his head may have actually concussed him - he could no longer focus on his target visually and presumably couldn't focus his chi, causing the blast to go off much closer than intended as his chi was unleashed.