What exactly is Combustionman's Tattoo?
This is semi-good source for Combustion Man. This is a good source for special bending techniques in general.

It states that his tattoo is a focal point for him to trigger explosions by focusing his chi. Seeing as it's a unique use of bending, I wouldn't be surprised if he can only bend from that spot simply due to complexity and a necessity for accuracy. 

Additionally, the idea of the "third eye" is taken from Hindu tradition. THe combustion bending tattoo's is located over this Chakra, with the tattoo being of the Eye of Shiva. P'Li's is a more wavy version, which may be related to her ability to bend the beam as well.

All the bending are generally related to some kind of martial art, so if that doesn't answer your question, I'd be glad to do some additional research. Let me know!

Also, thanks to Ryan for the comment. It prompted the extra link and the tattoo design section.